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rfc:rfc5646

Network Working Group A. Phillips, Ed. Request for Comments: 5646 Lab126 BCP: 47 M. Davis, Ed. Obsoletes: 4646 Google Category: Best Current Practice September 2009

                   Tags for Identifying Languages

Abstract

 This document describes the structure, content, construction, and
 semantics of language tags for use in cases where it is desirable to
 indicate the language used in an information object.  It also
 describes how to register values for use in language tags and the
 creation of user-defined extensions for private interchange.

Status of This Memo

 This document specifies an Internet Best Current Practices for the
 Internet Community, and requests discussion and suggestions for
 improvements.  Distribution of this memo is unlimited.

Copyright Notice

 Copyright (c) 2009 IETF Trust and the persons identified as the
 document authors.  All rights reserved.
 This document is subject to BCP 78 and the IETF Trust's Legal
 Provisions Relating to IETF Documents in effect on the date of
 publication of this document (http://trustee.ietf.org/license-info).
 Please review these documents carefully, as they describe your rights
 and restrictions with respect to this document.
 This document may contain material from IETF Documents or IETF
 Contributions published or made publicly available before November
 10, 2008.  The person(s) controlling the copyright in some of this
 material may not have granted the IETF Trust the right to allow
 modifications of such material outside the IETF Standards Process.
 Without obtaining an adequate license from the person(s) controlling
 the copyright in such materials, this document may not be modified
 outside the IETF Standards Process, and derivative works of it may
 not be created outside the IETF Standards Process, except to format
 it for publication as an RFC or to translate it into languages other
 than English.

Phillips & Davis Best Current Practice [Page 1] RFC 5646 Language Tags September 2009

Table of Contents

 1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  3
 2.  The Language Tag . . . . . . . . . . . . . . . . . . . . . . .  4
   2.1.  Syntax . . . . . . . . . . . . . . . . . . . . . . . . . .  4
     2.1.1.  Formatting of Language Tags  . . . . . . . . . . . . .  6
   2.2.  Language Subtag Sources and Interpretation . . . . . . . .  8
     2.2.1.  Primary Language Subtag . . . . . . . . . . . . . . . . 9
     2.2.2.  Extended Language Subtags  . . . . . . . . . . . . . . 11
     2.2.3.  Script Subtag  . . . . . . . . . . . . . . . . . . . . 12
     2.2.4.  Region Subtag  . . . . . . . . . . . . . . . . . . . . 13
     2.2.5.  Variant Subtags  . . . . . . . . . . . . . . . . . . . 15
     2.2.6.  Extension Subtags  . . . . . . . . . . . . . . . . . . 16
     2.2.7.  Private Use Subtags  . . . . . . . . . . . . . . . . . 18
     2.2.8.  Grandfathered and Redundant Registrations  . . . . . . 18
     2.2.9.  Classes of Conformance . . . . . . . . . . . . . . . . 19
 3.  Registry Format and Maintenance  . . . . . . . . . . . . . . . 21
   3.1.  Format of the IANA Language Subtag Registry  . . . . . . . 21
     3.1.1.  File Format  . . . . . . . . . . . . . . . . . . . . . 21
     3.1.2.  Record and Field Definitions . . . . . . . . . . . . . 23
     3.1.3.  Type Field . . . . . . . . . . . . . . . . . . . . . . 26
     3.1.4.  Subtag and Tag Fields  . . . . . . . . . . . . . . . . 26
     3.1.5.  Description Field  . . . . . . . . . . . . . . . . . . 26
     3.1.6.  Deprecated Field . . . . . . . . . . . . . . . . . . . 28
     3.1.7.  Preferred-Value Field  . . . . . . . . . . . . . . . . 28
     3.1.8.  Prefix Field . . . . . . . . . . . . . . . . . . . . . 31
     3.1.9.  Suppress-Script Field  . . . . . . . . . . . . . . . . 32
     3.1.10. Macrolanguage Field  . . . . . . . . . . . . . . . . . 32
     3.1.11. Scope Field  . . . . . . . . . . . . . . . . . . . . . 33
     3.1.12. Comments Field . . . . . . . . . . . . . . . . . . . . 34
   3.2.  Language Subtag Reviewer . . . . . . . . . . . . . . . . . 35
   3.3.  Maintenance of the Registry  . . . . . . . . . . . . . . . 35
   3.4.  Stability of IANA Registry Entries . . . . . . . . . . . . 36
   3.5.  Registration Procedure for Subtags . . . . . . . . . . . . 41
   3.6.  Possibilities for Registration . . . . . . . . . . . . . . 46
   3.7.  Extensions and the Extensions Registry . . . . . . . . . . 49
   3.8.  Update of the Language Subtag Registry . . . . . . . . . . 52
   3.9.  Applicability of the Subtag Registry . . . . . . . . . . . 52
 4.  Formation and Processing of Language Tags  . . . . . . . . . . 53
   4.1.  Choice of Language Tag . . . . . . . . . . . . . . . . . . 53
     4.1.1.  Tagging Encompassed Languages  . . . . . . . . . . . . 58
     4.1.2.  Using Extended Language Subtags  . . . . . . . . . . . 59
   4.2.  Meaning of the Language Tag  . . . . . . . . . . . . . . . 61
   4.3.  Lists of Languages . . . . . . . . . . . . . . . . . . . . 63
   4.4.  Length Considerations  . . . . . . . . . . . . . . . . . . 63
     4.4.1.  Working with Limited Buffer Sizes  . . . . . . . . . . 64
     4.4.2.  Truncation of Language Tags  . . . . . . . . . . . . . 65
   4.5.  Canonicalization of Language Tags  . . . . . . . . . . . . 66

Phillips & Davis Best Current Practice [Page 2] RFC 5646 Language Tags September 2009

   4.6.  Considerations for Private Use Subtags . . . . . . . . . . 68
 5.  IANA Considerations  . . . . . . . . . . . . . . . . . . . . . 69
   5.1.  Language Subtag Registry . . . . . . . . . . . . . . . . . 69
   5.2.  Extensions Registry  . . . . . . . . . . . . . . . . . . . 71
 6.  Security Considerations  . . . . . . . . . . . . . . . . . . . 71
 7.  Character Set Considerations . . . . . . . . . . . . . . . . . 72
 8.  Changes from RFC 4646  . . . . . . . . . . . . . . . . . . . . 73
 9.  References . . . . . . . . . . . . . . . . . . . . . . . . . . 76
   9.1.  Normative References . . . . . . . . . . . . . . . . . . . 76
   9.2.  Informative References . . . . . . . . . . . . . . . . . . 78
 Appendix A.  Examples of Language Tags (Informative) . . . . . . . 80
 Appendix B.  Examples of Registration Forms  . . . . . . . . . . . 82
 Appendix C.  Acknowledgements  . . . . . . . . . . . . . . . . . . 83

1. Introduction

 Human beings on our planet have, past and present, used a number of
 languages.  There are many reasons why one would want to identify the
 language used when presenting or requesting information.
 The language of an information item or a user's language preferences
 often need to be identified so that appropriate processing can be
 applied.  For example, the user's language preferences in a Web
 browser can be used to select Web pages appropriately.  Language
 information can also be used to select among tools (such as
 dictionaries) to assist in the processing or understanding of content
 in different languages.  Knowledge about the particular language used
 by some piece of information content might be useful or even required
 by some types of processing, for example, spell-checking, computer-
 synthesized speech, Braille transcription, or high-quality print
 renderings.
 One means of indicating the language used is by labeling the
 information content with an identifier or "tag".  These tags can also
 be used to specify the user's preferences when selecting information
 content or to label additional attributes of content and associated
 resources.
 Sometimes language tags are used to indicate additional language
 attributes of content.  For example, indicating specific information
 about the dialect, writing system, or orthography used in a document
 or resource may enable the user to obtain information in a form that
 they can understand, or it can be important in processing or
 rendering the given content into an appropriate form or style.
 This document specifies a particular identifier mechanism (the
 language tag) and a registration function for values to be used to

Phillips & Davis Best Current Practice [Page 3] RFC 5646 Language Tags September 2009

 form tags.  It also defines a mechanism for private use values and
 future extensions.
 This document replaces [RFC4646] (which obsoleted [RFC3066] which, in
 turn, replaced [RFC1766]).  This document, in combination with
 [RFC4647], comprises BCP 47.  For a list of changes in this document,
 see Section 8.
 The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
 "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
 document are to be interpreted as described in [RFC2119].

2. The Language Tag

 Language tags are used to help identify languages, whether spoken,
 written, signed, or otherwise signaled, for the purpose of
 communication.  This includes constructed and artificial languages
 but excludes languages not intended primarily for human
 communication, such as programming languages.

2.1. Syntax

 A language tag is composed from a sequence of one or more "subtags",
 each of which refines or narrows the range of language identified by
 the overall tag.  Subtags, in turn, are a sequence of alphanumeric
 characters (letters and digits), distinguished and separated from
 other subtags in a tag by a hyphen ("-", [Unicode] U+002D).
 There are different types of subtag, each of which is distinguished
 by length, position in the tag, and content: each subtag's type can
 be recognized solely by these features.  This makes it possible to
 extract and assign some semantic information to the subtags, even if
 the specific subtag values are not recognized.  Thus, a language tag
 processor need not have a list of valid tags or subtags (that is, a
 copy of some version of the IANA Language Subtag Registry) in order
 to perform common searching and matching operations.  The only
 exceptions to this ability to infer meaning from subtag structure are
 the grandfathered tags listed in the productions 'regular' and
 'irregular' below.  These tags were registered under [RFC3066] and
 are a fixed list that can never change.
 The syntax of the language tag in ABNF [RFC5234] is:

Language-Tag = langtag ; normal language tags

             / privateuse          ; private use tag
             / grandfathered       ; grandfathered tags

Phillips & Davis Best Current Practice [Page 4] RFC 5646 Language Tags September 2009

langtag = language

               ["-" script]
               ["-" region]
               *("-" variant)
               *("-" extension)
               ["-" privateuse]

language = 2*3ALPHA ; shortest ISO 639 code

               ["-" extlang]       ; sometimes followed by
                                   ; extended language subtags
             / 4ALPHA              ; or reserved for future use
             / 5*8ALPHA            ; or registered language subtag

extlang = 3ALPHA ; selected ISO 639 codes

  • 2("-" 3ALPHA) ; permanently reserved

script = 4ALPHA ; ISO 15924 code

region = 2ALPHA ; ISO 3166-1 code

             / 3DIGIT              ; UN M.49 code

variant = 5*8alphanum ; registered variants

             / (DIGIT 3alphanum)

extension = singleton 1*("-" (2*8alphanum))

                                   ; Single alphanumerics
                                   ; "x" reserved for private use

singleton = DIGIT ; 0 - 9

             / %x41-57             ; A - W
             / %x59-5A             ; Y - Z
             / %x61-77             ; a - w
             / %x79-7A             ; y - z

privateuse = "x" 1*("-" (1*8alphanum))

grandfathered = irregular ; non-redundant tags registered

             / regular             ; during the RFC 3066 era

irregular = "en-GB-oed" ; irregular tags do not match

             / "i-ami"             ; the 'langtag' production and
             / "i-bnn"             ; would not otherwise be
             / "i-default"         ; considered 'well-formed'
             / "i-enochian"        ; These tags are all valid,
             / "i-hak"             ; but most are deprecated
             / "i-klingon"         ; in favor of more modern
             / "i-lux"             ; subtags or subtag
             / "i-mingo"           ; combination

Phillips & Davis Best Current Practice [Page 5] RFC 5646 Language Tags September 2009

             / "i-navajo"
             / "i-pwn"
             / "i-tao"
             / "i-tay"
             / "i-tsu"
             / "sgn-BE-FR"
             / "sgn-BE-NL"
             / "sgn-CH-DE"

regular = "art-lojban" ; these tags match the 'langtag'

             / "cel-gaulish"       ; production, but their subtags
             / "no-bok"            ; are not extended language
             / "no-nyn"            ; or variant subtags: their meaning
             / "zh-guoyu"          ; is defined by their registration
             / "zh-hakka"          ; and all of these are deprecated
             / "zh-min"            ; in favor of a more modern
             / "zh-min-nan"        ; subtag or sequence of subtags
             / "zh-xiang"

alphanum = (ALPHA / DIGIT) ; letters and numbers

                      Figure 1: Language Tag ABNF
 For examples of language tags, see Appendix A.
 All subtags have a maximum length of eight characters.  Whitespace is
 not permitted in a language tag.  There is a subtlety in the ABNF
 production 'variant': a variant starting with a digit has a minimum
 length of four characters, while those starting with a letter have a
 minimum length of five characters.
 Although [RFC5234] refers to octets, the language tags described in
 this document are sequences of characters from the US-ASCII [ISO646]
 repertoire.  Language tags MAY be used in documents and applications
 that use other encodings, so long as these encompass the relevant
 part of the US-ASCII repertoire.  An example of this would be an XML
 document that uses the UTF-16LE [RFC2781] encoding of [Unicode].

2.1.1. Formatting of Language Tags

 At all times, language tags and their subtags, including private use
 and extensions, are to be treated as case insensitive: there exist
 conventions for the capitalization of some of the subtags, but these
 MUST NOT be taken to carry meaning.
 Thus, the tag "mn-Cyrl-MN" is not distinct from "MN-cYRL-mn" or "mN-
 cYrL-Mn" (or any other combination), and each of these variations

Phillips & Davis Best Current Practice [Page 6] RFC 5646 Language Tags September 2009

 conveys the same meaning: Mongolian written in the Cyrillic script as
 used in Mongolia.
 The ABNF syntax also does not distinguish between upper- and
 lowercase: the uppercase US-ASCII letters in the range 'A' through
 'Z' are always considered equivalent and mapped directly to their US-
 ASCII lowercase equivalents in the range 'a' through 'z'.  So the tag
 "I-AMI" is considered equivalent to that value "i-ami" in the
 'irregular' production.
 Although case distinctions do not carry meaning in language tags,
 consistent formatting and presentation of language tags will aid
 users.  The format of subtags in the registry is RECOMMENDED as the
 form to use in language tags.  This format generally corresponds to
 the common conventions for the various ISO standards from which the
 subtags are derived.
 These conventions include:
 o  [ISO639-1] recommends that language codes be written in lowercase
    ('mn' Mongolian).
 o  [ISO15924] recommends that script codes use lowercase with the
    initial letter capitalized ('Cyrl' Cyrillic).
 o  [ISO3166-1] recommends that country codes be capitalized ('MN'
    Mongolia).
 An implementation can reproduce this format without accessing the
 registry as follows.  All subtags, including extension and private
 use subtags, use lowercase letters with two exceptions: two-letter
 and four-letter subtags that neither appear at the start of the tag
 nor occur after singletons.  Such two-letter subtags are all
 uppercase (as in the tags "en-CA-x-ca" or "sgn-BE-FR") and four-
 letter subtags are titlecase (as in the tag "az-Latn-x-latn").
 Note: Case folding of ASCII letters in certain locales, unless
 carefully handled, sometimes produces non-ASCII character values.
 The Unicode Character Database file "SpecialCasing.txt"
 [SpecialCasing] defines the specific cases that are known to cause
 problems with this.  In particular, the letter 'i' (U+0069) in
 Turkish and Azerbaijani is uppercased to U+0130 (LATIN CAPITAL LETTER
 I WITH DOT ABOVE).  Implementers SHOULD specify a locale-neutral
 casing operation to ensure that case folding of subtags does not
 produce this value, which is illegal in language tags.  For example,
 if one were to uppercase the region subtag 'in' using Turkish locale
 rules, the sequence U+0130 U+004E would result, instead of the
 expected 'IN'.

Phillips & Davis Best Current Practice [Page 7] RFC 5646 Language Tags September 2009

2.2. Language Subtag Sources and Interpretation

 The namespace of language tags and their subtags is administered by
 the Internet Assigned Numbers Authority (IANA) according to the rules
 in Section 5 of this document.  The Language Subtag Registry
 maintained by IANA is the source for valid subtags: other standards
 referenced in this section provide the source material for that
 registry.
 Terminology used in this document:
 o  "Tag" refers to a complete language tag, such as "sr-Latn-RS" or
    "az-Arab-IR".  Examples of tags in this document are enclosed in
    double-quotes ("en-US").
 o  "Subtag" refers to a specific section of a tag, delimited by a
    hyphen, such as the subtags 'zh', 'Hant', and 'CN' in the tag "zh-
    Hant-CN".  Examples of subtags in this document are enclosed in
    single quotes ('Hant').
 o  "Code" refers to values defined in external standards (and that
    are used as subtags in this document).  For example, 'Hant' is an
    [ISO15924] script code that was used to define the 'Hant' script
    subtag for use in a language tag.  Examples of codes in this
    document are enclosed in single quotes ('en', 'Hant').
 Language tags are designed so that each subtag type has unique length
 and content restrictions.  These make identification of the subtag's
 type possible, even if the content of the subtag itself is
 unrecognized.  This allows tags to be parsed and processed without
 reference to the latest version of the underlying standards or the
 IANA registry and makes the associated exception handling when
 parsing tags simpler.
 Some of the subtags in the IANA registry do not come from an
 underlying standard.  These can only appear in specific positions in
 a tag: they can only occur as primary language subtags or as variant
 subtags.
 Sequences of private use and extension subtags MUST occur at the end
 of the sequence of subtags and MUST NOT be interspersed with subtags
 defined elsewhere in this document.  These sequences are introduced
 by single-character subtags, which are reserved as follows:
 o  The single-letter subtag 'x' introduces a sequence of private use
    subtags.  The interpretation of any private use subtag is defined

Phillips & Davis Best Current Practice [Page 8] RFC 5646 Language Tags September 2009

    solely by private agreement and is not defined by the rules in
    this section or in any standard or registry defined in this
    document.
 o  The single-letter subtag 'i' is used by some grandfathered tags,
    such as "i-default", where it always appears in the first position
    and cannot be confused with an extension.
 o  All other single-letter and single-digit subtags are reserved to
    introduce standardized extension subtag sequences as described in
    Section 3.7.

2.2.1. Primary Language Subtag

 The primary language subtag is the first subtag in a language tag and
 cannot be omitted, with two exceptions:
 o  The single-character subtag 'x' as the primary subtag indicates
    that the language tag consists solely of subtags whose meaning is
    defined by private agreement.  For example, in the tag "x-fr-CH",
    the subtags 'fr' and 'CH' do not represent the French language or
    the country of Switzerland (or any other value in the IANA
    registry) unless there is a private agreement in place to do so.
    See Section 4.6.
 o  The single-character subtag 'i' is used by some grandfathered tags
    (see Section 2.2.8) such as "i-klingon" and "i-bnn".  (Other
    grandfathered tags have a primary language subtag in their first
    position.)
 The following rules apply to the primary language subtag:
 1.  Two-character primary language subtags were defined in the IANA
     registry according to the assignments found in the standard "ISO
     639-1:2002, Codes for the representation of names of languages --
     Part 1: Alpha-2 code" [ISO639-1], or using assignments
     subsequently made by the ISO 639-1 registration authority (RA) or
     governing standardization bodies.
 2.  Three-character primary language subtags in the IANA registry
     were defined according to the assignments found in one of these
     additional ISO 639 parts or assignments subsequently made by the
     relevant ISO 639 registration authorities or governing
     standardization bodies:
     A.  "ISO 639-2:1998 - Codes for the representation of names of
         languages -- Part 2: Alpha-3 code - edition 1" [ISO639-2]

Phillips & Davis Best Current Practice [Page 9] RFC 5646 Language Tags September 2009

     B.  "ISO 639-3:2007 - Codes for the representation of names of
         languages -- Part 3: Alpha-3 code for comprehensive coverage
         of languages" [ISO639-3]
     C.  "ISO 639-5:2008 - Codes for the representation of names of
         languages -- Part 5: Alpha-3 code for language families and
         groups" [ISO639-5]
 3.  The subtags in the range 'qaa' through 'qtz' are reserved for
     private use in language tags.  These subtags correspond to codes
     reserved by ISO 639-2 for private use.  These codes MAY be used
     for non-registered primary language subtags (instead of using
     private use subtags following 'x-').  Please refer to Section 4.6
     for more information on private use subtags.
 4.  Four-character language subtags are reserved for possible future
     standardization.
 5.  Any language subtags of five to eight characters in length in the
     IANA registry were defined via the registration process in
     Section 3.5 and MAY be used to form the primary language subtag.
     An example of what such a registration might include is the
     grandfathered IANA registration "i-enochian".  The subtag
     'enochian' could be registered in the IANA registry as a primary
     language subtag (assuming that ISO 639 does not register this
     language first), making tags such as "enochian-AQ" and "enochian-
     Latn" valid.
     At the time this document was created, there were no examples of
     this kind of subtag.  Future registrations of this type are
     discouraged: an attempt to register any new proposed primary
     language MUST be made to the ISO 639 registration authority.
     Proposals rejected by the ISO 639 registration authority are
     unlikely to meet the criteria for primary language subtags and
     are thus unlikely to be registered.
 6.  Other values MUST NOT be assigned to the primary subtag except by
     revision or update of this document.
 When languages have both an ISO 639-1 two-character code and a three-
 character code (assigned by ISO 639-2, ISO 639-3, or ISO 639-5), only
 the ISO 639-1 two-character code is defined in the IANA registry.
 When a language has no ISO 639-1 two-character code and the ISO
 639-2/T (Terminology) code and the ISO 639-2/B (Bibliographic) code
 for that language differ, only the Terminology code is defined in the
 IANA registry.  At the time this document was created, all languages
 that had both kinds of three-character codes were also assigned a

Phillips & Davis Best Current Practice [Page 10] RFC 5646 Language Tags September 2009

 two-character code; it is expected that future assignments of this
 nature will not occur.
 In order to avoid instability in the canonical form of tags, if a
 two-character code is added to ISO 639-1 for a language for which a
 three-character code was already included in either ISO 639-2 or ISO
 639-3, the two-character code MUST NOT be registered.  See
 Section 3.4.
 For example, if some content were tagged with 'haw' (Hawaiian), which
 currently has no two-character code, the tag would not need to be
 changed if ISO 639-1 were to assign a two-character code to the
 Hawaiian language at a later date.
 To avoid these problems with versioning and subtag choice (as
 experienced during the transition between RFC 1766 and RFC 3066), as
 well as to ensure the canonical nature of subtags defined by this
 document, the ISO 639 Registration Authority Joint Advisory Committee
 (ISO 639/RA-JAC) has included the following statement in
 [iso639.prin]:
    "A language code already in ISO 639-2 at the point of freezing ISO
    639-1 shall not later be added to ISO 639-1.  This is to ensure
    consistency in usage over time, since users are directed in
    Internet applications to employ the alpha-3 code when an alpha-2
    code for that language is not available."

2.2.2. Extended Language Subtags

 Extended language subtags are used to identify certain specially
 selected languages that, for various historical and compatibility
 reasons, are closely identified with or tagged using an existing
 primary language subtag.  Extended language subtags are always used
 with their enclosing primary language subtag (indicated with a
 'Prefix' field in the registry) when used to form the language tag.
 All languages that have an extended language subtag in the registry
 also have an identical primary language subtag record in the
 registry.  This primary language subtag is RECOMMENDED for forming
 the language tag.  The following rules apply to the extended language
 subtags:
 1.  Extended language subtags consist solely of three-letter subtags.
     All extended language subtag records defined in the registry were
     defined according to the assignments found in [ISO639-3].
     Language collections and groupings, such as defined in
     [ISO639-5], are specifically excluded from being extended
     language subtags.

Phillips & Davis Best Current Practice [Page 11] RFC 5646 Language Tags September 2009

 2.  Extended language subtag records MUST include exactly one
     'Prefix' field indicating an appropriate subtag or sequence of
     subtags for that extended language subtag.
 3.  Extended language subtag records MUST include a 'Preferred-
     Value'.  The 'Preferred-Value' and 'Subtag' fields MUST be
     identical.
 4.  Although the ABNF production 'extlang' permits up to three
     extended language tags in the language tag, extended language
     subtags MUST NOT include another extended language subtag in
     their 'Prefix'.  That is, the second and third extended language
     subtag positions in a language tag are permanently reserved and
     tags that include those subtags in that position are, and will
     always remain, invalid.
 For example, the macrolanguage Chinese ('zh') encompasses a number of
 languages.  For compatibility reasons, each of these languages has
 both a primary and extended language subtag in the registry.  A few
 selected examples of these include Gan Chinese ('gan'), Cantonese
 Chinese ('yue'), and Mandarin Chinese ('cmn').  Each is encompassed
 by the macrolanguage 'zh' (Chinese).  Therefore, they each have the
 prefix "zh" in their registry records.  Thus, Gan Chinese is
 represented with tags beginning "zh-gan" or "gan", Cantonese with
 tags beginning either "yue" or "zh-yue", and Mandarin Chinese with
 "zh-cmn" or "cmn".  The language subtag 'zh' can still be used
 without an extended language subtag to label a resource as some
 unspecified variety of Chinese, while the primary language subtag
 ('gan', 'yue', 'cmn') is preferred to using the extended language
 form ("zh-gan", "zh-yue", "zh-cmn").

2.2.3. Script Subtag

 Script subtags are used to indicate the script or writing system
 variations that distinguish the written forms of a language or its
 dialects.  The following rules apply to the script subtags:
 1.  Script subtags MUST follow any primary and extended language
     subtags and MUST precede any other type of subtag.
 2.  Script subtags consist of four letters and were defined according
     to the assignments found in [ISO15924] ("Information and
     documentation -- Codes for the representation of names of
     scripts"), or subsequently assigned by the ISO 15924 registration
     authority or governing standardization bodies.  Only codes
     assigned by ISO 15924 will be considered for registration.

Phillips & Davis Best Current Practice [Page 12] RFC 5646 Language Tags September 2009

 3.  The script subtags 'Qaaa' through 'Qabx' are reserved for private
     use in language tags.  These subtags correspond to codes reserved
     by ISO 15924 for private use.  These codes MAY be used for non-
     registered script values.  Please refer to Section 4.6 for more
     information on private use subtags.
 4.  There MUST be at most one script subtag in a language tag, and
     the script subtag SHOULD be omitted when it adds no
     distinguishing value to the tag or when the primary or extended
     language subtag's record in the subtag registry includes a
     'Suppress-Script' field listing the applicable script subtag.
 For example: "sr-Latn" represents Serbian written using the Latin
 script.

2.2.4. Region Subtag

 Region subtags are used to indicate linguistic variations associated
 with or appropriate to a specific country, territory, or region.
 Typically, a region subtag is used to indicate variations such as
 regional dialects or usage, or region-specific spelling conventions.
 It can also be used to indicate that content is expressed in a way
 that is appropriate for use throughout a region, for instance,
 Spanish content tailored to be useful throughout Latin America.
 The following rules apply to the region subtags:
 1.  Region subtags MUST follow any primary language, extended
     language, or script subtags and MUST precede any other type of
     subtag.
 2.  Two-letter region subtags were defined according to the
     assignments found in [ISO3166-1] ("Codes for the representation
     of names of countries and their subdivisions -- Part 1: Country
     codes"), using the list of alpha-2 country codes or using
     assignments subsequently made by the ISO 3166-1 maintenance
     agency or governing standardization bodies.  In addition, the
     codes that are "exceptionally reserved" (as opposed to
     "assigned") in ISO 3166-1 were also defined in the registry, with
     the exception of 'UK', which is an exact synonym for the assigned
     code 'GB'.
 3.  The region subtags 'AA', 'QM'-'QZ', 'XA'-'XZ', and 'ZZ' are
     reserved for private use in language tags.  These subtags
     correspond to codes reserved by ISO 3166 for private use.  These
     codes MAY be used for private use region subtags (instead of
     using a private use subtag sequence).  Please refer to
     Section 4.6 for more information on private use subtags.

Phillips & Davis Best Current Practice [Page 13] RFC 5646 Language Tags September 2009

 4.  Three-character region subtags consist solely of digit (number)
     characters and were defined according to the assignments found in
     the UN Standard Country or Area Codes for Statistical  Use
     [UN_M.49] or assignments subsequently made by the governing
     standards body.  Not all of the UN M.49 codes are defined in the
     IANA registry.  The following rules define which codes are
     entered into the registry as valid subtags:
     A.  UN numeric codes assigned to 'macro-geographical
         (continental)' or sub-regions MUST be registered in the
         registry.  These codes are not associated with an assigned
         ISO 3166-1 alpha-2 code and represent supra-national areas,
         usually covering more than one nation, state, province, or
         territory.
     B.  UN numeric codes for 'economic groupings' or 'other
         groupings' MUST NOT be registered in the IANA registry and
         MUST NOT be used to form language tags.
     C.  When ISO 3166-1 reassigns a code formerly used for one
         country or area to another country or area and that code
         already is present in the registry, the UN numeric code for
         that country or area MUST be registered in the registry as
         described in Section 3.4 and MUST be used to form language
         tags that represent the country or region for which it is
         defined (rather than the recycled ISO 3166-1 code).
     D.  UN numeric codes for countries or areas for which there is an
         associated ISO 3166-1 alpha-2 code in the registry MUST NOT
         be entered into the registry and MUST NOT be used to form
         language tags.  Note that the ISO 3166-based subtag in the
         registry MUST actually be associated with the UN M.49 code in
         question.
     E.  For historical reasons, the UN numeric code 830 (Channel
         Islands), which was not registered at the time this document
         was adopted and had, at that time, no corresponding ISO
         3166-1 code, MAY be entered into the IANA registry via the
         process described in Section 3.5, provided no ISO 3166-1 code
         with that exact meaning has been previously registered.
     F.  All other UN numeric codes for countries or areas that do not
         have an associated ISO 3166-1 alpha-2 code MUST NOT be
         entered into the registry and MUST NOT be used to form
         language tags.  For more information about these codes, see
         Section 3.4.

Phillips & Davis Best Current Practice [Page 14] RFC 5646 Language Tags September 2009

 5.  The alphanumeric codes in Appendix X of the UN document MUST NOT
     be entered into the registry and MUST NOT be used to form
     language tags.  (At the time this document was created, these
     values matched the ISO 3166-1 alpha-2 codes.)
 6.  There MUST be at most one region subtag in a language tag and the
     region subtag MAY be omitted, as when it adds no distinguishing
     value to the tag.
 For example:
    "de-AT" represents German ('de') as used in Austria ('AT').
    "sr-Latn-RS" represents Serbian ('sr') written using Latin script
    ('Latn') as used in Serbia ('RS').
    "es-419" represents Spanish ('es') appropriate to the UN-defined
    Latin America and Caribbean region ('419').

2.2.5. Variant Subtags

 Variant subtags are used to indicate additional, well-recognized
 variations that define a language or its dialects that are not
 covered by other available subtags.  The following rules apply to the
 variant subtags:
 1.  Variant subtags MUST follow any primary language, extended
     language, script, or region subtags and MUST precede any
     extension or private use subtag sequences.
 2.  Variant subtags, as a collection, are not associated with any
     particular external standard.  The meaning of variant subtags in
     the registry is defined in the course of the registration process
     defined in Section 3.5.  Note that any particular variant subtag
     might be associated with some external standard.  However,
     association with a standard is not required for registration.
 3.  More than one variant MAY be used to form the language tag.
 4.  Variant subtags MUST be registered with IANA according to the
     rules in Section 3.5 of this document before being used to form
     language tags.  In order to distinguish variants from other types
     of subtags, registrations MUST meet the following length and
     content restrictions:
     1.  Variant subtags that begin with a letter (a-z, A-Z) MUST be
         at least five characters long.

Phillips & Davis Best Current Practice [Page 15] RFC 5646 Language Tags September 2009

     2.  Variant subtags that begin with a digit (0-9) MUST be at
         least four characters long.
 5.  The same variant subtag MUST NOT be used more than once within a
     language tag.
  • For example, the tag "de-DE-1901-1901" is not valid.
 Variant subtag records in the Language Subtag Registry MAY include
 one or more 'Prefix' (Section 3.1.8) fields.  Each 'Prefix' indicates
 a suitable sequence of subtags for forming (with other subtags, as
 appropriate) a language tag when using the variant.
 Most variants that share a prefix are mutually exclusive.  For
 example, the German orthographic variations '1996' and '1901' SHOULD
 NOT be used in the same tag, as they represent the dates of different
 spelling reforms.  A variant that can meaningfully be used in
 combination with another variant SHOULD include a 'Prefix' field in
 its registry record that lists that other variant.  For example, if
 another German variant 'example' were created that made sense to use
 with '1996', then 'example' should include two 'Prefix' fields: "de"
 and "de-1996".
 For example:
    "sl-nedis" represents the Natisone or Nadiza dialect of Slovenian.
    "de-CH-1996" represents German as used in Switzerland and as
    written using the spelling reform beginning in the year 1996 C.E.

2.2.6. Extension Subtags

 Extensions provide a mechanism for extending language tags for use in
 various applications.  They are intended to identify information that
 is commonly used in association with languages or language tags but
 that is not part of language identification.  See Section 3.7.  The
 following rules apply to extensions:
 1.  An extension MUST follow at least a primary language subtag.
     That is, a language tag cannot begin with an extension.
     Extensions extend language tags, they do not override or replace
     them.  For example, "a-value" is not a well-formed language tag,
     while "de-a-value" is.  Note that extensions cannot be used in
     tags that are entirely private use (that is, tags starting with
     "x-").

Phillips & Davis Best Current Practice [Page 16] RFC 5646 Language Tags September 2009

 2.  Extension subtags are separated from the other subtags defined in
     this document by a single-character subtag (called a
     "singleton").  The singleton MUST be one allocated to a
     registration authority via the mechanism described in Section 3.7
     and MUST NOT be the letter 'x', which is reserved for private use
     subtag sequences.
 3.  Each singleton subtag MUST appear at most one time in each tag
     (other than as a private use subtag).  That is, singleton subtags
     MUST NOT be repeated.  For example, the tag "en-a-bbb-a-ccc" is
     invalid because the subtag 'a' appears twice.  Note that the tag
     "en-a-bbb-x-a-ccc" is valid because the second appearance of the
     singleton 'a' is in a private use sequence.
 4.  Extension subtags MUST meet whatever requirements are set by the
     document that defines their singleton prefix and whatever
     requirements are provided by the maintaining authority.  Note
     that there might not be a registry of these subtags and
     validating processors are not required to validate extensions.
 5.  Each extension subtag MUST be from two to eight characters long
     and consist solely of letters or digits, with each subtag
     separated by a single '-'.  Case distinctions are ignored in
     extensions (as with any language subtag) and normalized subtags
     of this type are expected to be in lowercase.
 6.  Each singleton MUST be followed by at least one extension subtag.
     For example, the tag "tlh-a-b-foo" is invalid because the first
     singleton 'a' is followed immediately by another singleton 'b'.
 7.  Extension subtags MUST follow all primary language, extended
     language, script, region, and variant subtags in a tag and MUST
     precede any private use subtag sequences.
 8.  All subtags following the singleton and before another singleton
     are part of the extension.  Example: In the tag "fr-a-Latn", the
     subtag 'Latn' does not represent the script subtag 'Latn' defined
     in the IANA Language Subtag Registry.  Its meaning is defined by
     the extension 'a'.
 9.  In the event that more than one extension appears in a single
     tag, the tag SHOULD be canonicalized as described in Section 4.5,
     by ordering the various extension sequences into case-insensitive
     ASCII order.
 For example, if an extension were defined for the singleton 'r' and
 it defined the subtags shown, then the following tag would be a valid
 example: "en-Latn-GB-boont-r-extended-sequence-x-private".

Phillips & Davis Best Current Practice [Page 17] RFC 5646 Language Tags September 2009

2.2.7. Private Use Subtags

 Private use subtags are used to indicate distinctions in language
 that are important in a given context by private agreement.  The
 following rules apply to private use subtags:
 1.  Private use subtags are separated from the other subtags defined
     in this document by the reserved single-character subtag 'x'.
 2.  Private use subtags MUST conform to the format and content
     constraints defined in the ABNF for all subtags; that is, they
     MUST consist solely of letters and digits and not exceed eight
     characters in length.
 3.  Private use subtags MUST follow all primary language, extended
     language, script, region, variant, and extension subtags in the
     tag.  Another way of saying this is that all subtags following
     the singleton 'x' MUST be considered private use.  Example: The
     subtag 'US' in the tag "en-x-US" is a private use subtag.
 4.  A tag MAY consist entirely of private use subtags.
 5.  No source is defined for private use subtags.  Use of private use
     subtags is by private agreement only.
 6.  Private use subtags are NOT RECOMMENDED where alternatives exist
     or for general interchange.  See Section 4.6 for more information
     on private use subtag choice.
 For example, suppose a group of scholars is studying some texts in
 medieval Greek.  They might agree to use some collection of private
 use subtags to identify different styles of writing in the texts.
 For example, they might use 'el-x-koine' for documents in the
 "common" style while using 'el-x-attic' for other documents that
 mimic the Attic style.  These subtags would not be recognized by
 outside processes or systems, but might be useful in categorizing
 various texts for study by those in the group.
 In the registry, there are also subtags derived from codes reserved
 by ISO 639, ISO 15924, or ISO 3166 for private use.  Do not confuse
 these with private use subtag sequences following the subtag 'x'.
 See Section 4.6.

2.2.8. Grandfathered and Redundant Registrations

 Prior to RFC 4646, whole language tags were registered according to
 the rules in RFC 1766 and/or RFC 3066.  All of these registered tags
 remain valid as language tags.

Phillips & Davis Best Current Practice [Page 18] RFC 5646 Language Tags September 2009

 Many of these registered tags were made redundant by the advent of
 either RFC 4646 or this document.  A redundant tag is a grandfathered
 registration whose individual subtags appear with the same semantic
 meaning in the registry.  For example, the tag "zh-Hant" (Traditional
 Chinese) can now be composed from the subtags 'zh' (Chinese) and
 'Hant' (Han script traditional variant).  These redundant tags are
 maintained in the registry as records of type 'redundant', mostly as
 a matter of historical curiosity.
 The remainder of the previously registered tags are "grandfathered".
 These tags are classified into two groups: 'regular' and 'irregular'.
 Grandfathered tags that (appear to) match the 'langtag' production in
 Figure 1 are considered 'regular' grandfathered tags.  These tags
 contain one or more subtags that either do not individually appear in
 the registry or appear but with a different semantic meaning: each
 tag, in its entirety, represents a language or collection of
 languages.
 Grandfathered tags that do not match the 'langtag' production in the
 ABNF and would otherwise be invalid are considered 'irregular'
 grandfathered tags.  With the exception of "en-GB-oed", which is a
 variant of "en-GB", each of them, in its entirety, represents a
 language.
 Many of the grandfathered tags have been superseded by the subsequent
 addition of new subtags: each superseded record contains a
 'Preferred-Value' field that ought to be used to form language tags
 representing that value.  For example, the tag "art-lojban" is
 superseded by the primary language subtag 'jbo'.

2.2.9. Classes of Conformance

 Implementations sometimes need to describe their capabilities with
 regard to the rules and practices described in this document.  Tags
 can be checked or verified in a number of ways, but two particular
 classes of tag conformance are formally defined here.
 A tag is considered "well-formed" if it conforms to the ABNF
 (Section 2.1).  Language tags may be well-formed in terms of syntax
 but not valid in terms of content.  However, many operations
 involving language tags work well without knowing anything about the
 meaning or validity of the subtags.
 A tag is considered "valid" if it satisfies these conditions:
 o  The tag is well-formed.

Phillips & Davis Best Current Practice [Page 19] RFC 5646 Language Tags September 2009

 o  Either the tag is in the list of grandfathered tags or all of its
    primary language, extended language, script, region, and variant
    subtags appear in the IANA Language Subtag Registry as of the
    particular registry date.
 o  There are no duplicate variant subtags.
 o  There are no duplicate singleton (extension) subtags.
 Note that a tag's validity depends on the date of the registry used
 to validate the tag.  A more recent copy of the registry might
 contain a subtag that an older version does not.
 A tag is considered valid for a given extension (Section 3.7) (as of
 a particular version, revision, and date) if it meets the criteria
 for "valid" above and also satisfies this condition:
    Each subtag used in the extension part of the tag is valid
    according to the extension.
 Older specifications or language tag implementations sometimes
 reference [RFC3066].  A wider array of tags was considered well-
 formed under that document.  Any tags that were valid for use under
 RFC 3066 are both well-formed and valid under this document's syntax;
 only invalid or illegal tags were well-formed under the earlier
 definition but no longer are.  The language tag syntax under RFC 3066
 was:
     obs-language-tag = primary-subtag *( "-" subtag )
     primary-subtag   = 1*8ALPHA
     subtag           = 1*8(ALPHA / DIGIT)
                Figure 2: RFC 3066 Language Tag Syntax
 Subtags designated for private use as well as private use sequences
 introduced by the 'x' subtag are available for cases in which no
 assigned subtags are available and registration is not a suitable
 option.  For example, one might use a tag such as "no-QQ", where 'QQ'
 is one of a range of private use ISO 3166-1 codes to indicate an
 otherwise undefined region.  Users MUST NOT assign language tags that
 use subtags that do not appear in the registry other than in private
 use sequences (such as the subtag 'personal' in the tag "en-x-
 personal").  Besides not being valid, the user also risks collision
 with a future possible assignment or registrations.
 Note well: although the 'Language-Tag' production appearing in this
 document is functionally equivalent to the one in [RFC4646], it has

Phillips & Davis Best Current Practice [Page 20] RFC 5646 Language Tags September 2009

 been changed to prevent certain errors in well-formedness arising
 from the old 'grandfathered' production.

3. Registry Format and Maintenance

 The IANA Language Subtag Registry ("the registry") contains a
 comprehensive list of all of the subtags valid in language tags.
 This allows implementers a straightforward and reliable way to
 validate language tags.  The registry will be maintained so that,
 except for extension subtags, it is possible to validate all of the
 subtags that appear in a language tag under the provisions of this
 document or its revisions or successors.  In addition, the meaning of
 the various subtags will be unambiguous and stable over time.  (The
 meaning of private use subtags, of course, is not defined by the
 registry.)
 This section defines the registry along with the maintenance and
 update procedures associated with it, as well as a registry for
 extensions to language tags (Section 3.7).

3.1. Format of the IANA Language Subtag Registry

 The IANA Language Subtag Registry is a machine-readable file in the
 format described in this section, plus copies of the registration
 forms approved in accordance with the process described in
 Section 3.5.
 The existing registration forms for grandfathered and redundant tags
 taken from RFC 3066 have been maintained as part of the obsolete RFC
 3066 registry.  The subtags added to the registry by either [RFC4645]
 or [RFC5645] do not have separate registration forms (so no forms are
 archived for these additions).

3.1.1. File Format

 The registry is a [Unicode] text file and consists of a series of
 records in a format based on "record-jar" (described in
 [record-jar]).  Each record, in turn, consists of a series of fields
 that describe the various subtags and tags.  The actual registry file
 is encoded using the UTF-8 [RFC3629] character encoding.
 Each field can be considered a single, logical line of characters.
 Each field contains a "field-name" and a "field-body".  These are
 separated by a "field-separator".  The field-separator is a COLON
 character (U+003A) plus any surrounding whitespace.  Each field is
 terminated by the newline sequence CRLF.  The text in each field MUST
 be in Unicode Normalization Form C (NFC).

Phillips & Davis Best Current Practice [Page 21] RFC 5646 Language Tags September 2009

 A collection of fields forms a "record".  Records are separated by
 lines containing only the sequence "%%" (U+0025 U+0025).
 Although fields are logically a single line of text, each line of
 text in the file format is limited to 72 bytes in length.  To
 accommodate this, the field-body can be split into a multiple-line
 representation; this is called "folding".  Folding is done according
 to customary conventions for line-wrapping.  This is typically on
 whitespace boundaries, but can occur between other characters when
 the value does not include spaces, such as when a language does not
 use whitespace between words.  In any event, there MUST NOT be breaks
 inside a multibyte UTF-8 sequence or in the middle of a combining
 character sequence.  For more information, see [UAX14].
 Although the file format uses the Unicode character set and the file
 itself is encoded using the UTF-8 encoding, fields are restricted to
 the printable characters from the US-ASCII [ISO646] repertoire unless
 otherwise indicated in the description of a specific field
 (Section 3.1.2).
 The format of the registry is described by the following ABNF
 [RFC5234].  Character numbers (code points) are taken from Unicode,
 and terminals in the ABNF productions are in terms of characters
 rather than bytes.
 registry   = record *("%%" CRLF record)
 record     = 1*field
 field      = ( field-name field-sep field-body CRLF )
 field-name = (ALPHA / DIGIT) [*(ALPHA / DIGIT / "-") (ALPHA / DIGIT)]
 field-sep  = *SP ":" *SP
 field-body = *([[*SP CRLF] 1*SP] 1*CHARS)
 CHARS      = (%x21-10FFFF)      ; Unicode code points
                    Figure 3: Registry Format ABNF
 The sequence '..'  (U+002E U+002E) in a field-body denotes a range of
 values.  Such a range represents all subtags of the same length that
 are in alphabetic or numeric order within that range, including the
 values explicitly mentioned.  For example, 'a..c' denotes the values
 'a', 'b', and 'c', and '11..13' denotes the values '11', '12', and
 '13'.
 All fields whose field-body contains a date value use the "full-date"
 format specified in [RFC3339].  For example, "2004-06-28" represents
 June 28, 2004, in the Gregorian calendar.

Phillips & Davis Best Current Practice [Page 22] RFC 5646 Language Tags September 2009

3.1.2. Record and Field Definitions

 There are three types of records in the registry: "File-Date",
 "Subtag", and "Tag".
 The first record in the registry is always the "File-Date" record.
 This record occurs only once in the file and contains a single field
 whose field-name is "File-Date".  The field-body of this record
 contains a date (see Section 5.1), making it possible to easily
 recognize different versions of the registry.
 File-Date: 2004-06-28
 %%
               Figure 4: Example of the File-Date Record
 Subsequent records contain multiple fields and represent information
 about either subtags or tags.  Both types of records have an
 identical structure, except that "Subtag" records contain a field
 with a field-name of "Subtag", while, unsurprisingly, "Tag" records
 contain a field with a field-name of "Tag".  Field-names MUST NOT
 occur more than once per record, with the exception of the
 'Description', 'Comments', and 'Prefix' fields.
 Each record MUST contain at least one of each of the following
 fields:
 o  'Type'
  • Type's field-body MUST consist of one of the following strings:

"language", "extlang", "script", "region", "variant",

       "grandfathered", and "redundant"; it denotes the type of tag or
       subtag.
 o  Either 'Subtag' or 'Tag'
  • Subtag's field-body contains the subtag being defined. This

field MUST appear in all records whose 'Type' has one of these

       values: "language", "extlang", "script", "region", or
       "variant".
  • Tag's field-body contains a complete language tag. This field

MUST appear in all records whose 'Type' has one of these

       values: "grandfathered" or "redundant".  If the 'Type' is
       "grandfathered", then the 'Tag' field-body will be one of the
       tags listed in either the 'regular' or 'irregular' production
       found in Section 2.1.

Phillips & Davis Best Current Practice [Page 23] RFC 5646 Language Tags September 2009

 o  'Description'
  • Description's field-body contains a non-normative description

of the subtag or tag.

 o  'Added'
  • Added's field-body contains the date the record was registered

or, in the case of grandfathered or redundant tags, the date

       the corresponding tag was registered under the rules of
       [RFC1766] or [RFC3066].
 Each record MAY also contain the following fields:
 o  'Deprecated'
  • Deprecated's field-body contains the date the record was

deprecated. In some cases, this value is earlier than that of

       the 'Added' field in the same record.  That is, the date of
       deprecation preceded the addition of the record to the
       registry.
 o  'Preferred-Value'
  • Preferred-Value's field-body contains a canonical mapping from

this record's value to a modern equivalent that is preferred in

       its place.  Depending on the value of the 'Type' field, this
       value can take different forms:
       +  For fields of type 'language', 'Preferred-Value' contains
          the primary language subtag that is preferred when forming
          the language tag.
       +  For fields of type 'script', 'region', or 'variant',
          'Preferred-Value' contains the subtag of the same type that
          is preferred for forming the language tag.
       +  For fields of type 'extlang', 'grandfathered', or
          'redundant', 'Preferred-Value' contains an "extended
          language range" [RFC4647] that is preferred for forming the
          language tag.  That is, the preferred language tag will
          contain, in order, each of the subtags that appears in the
          'Preferred-Value'; additional fields can be included in a
          language tag, as described elsewhere in this document.  For
          example, the replacement for the grandfathered tag "zh-min-
          nan" (Min Nan Chinese) is "nan", which can be used as the

Phillips & Davis Best Current Practice [Page 24] RFC 5646 Language Tags September 2009

          basis for tags such as "nan-Hant" or "nan-TW" (note that the
          extended language subtag form such as "zh-nan-Hant" or "zh-
          nan-TW" can also be used).
 o  'Prefix'
  • Prefix's field-body contains a valid language tag that is

RECOMMENDED as one possible prefix to this record's subtag.

       This field MAY appear in records whose 'Type' field-body is
       either 'extlang' or 'variant' (it MUST NOT appear in any other
       record type).
 o  'Suppress-Script'
  • Suppress-Script's field-body contains a script subtag that

SHOULD NOT be used to form language tags with the associated

       primary or extended language subtag.  This field MUST appear
       only in records whose 'Type' field-body is 'language' or
       'extlang'.  See Section 4.1.
 o  'Macrolanguage'
  • Macrolanguage's field-body contains a primary language subtag

defined by ISO 639 as the "macrolanguage" that encompasses this

       language subtag.  This field MUST appear only in records whose
       'Type' field-body is either 'language' or 'extlang'.
 o  'Scope'
  • Scope's field-body contains information about a primary or

extended language subtag indicating the type of language code

       according to ISO 639.  The values permitted in this field are
       "macrolanguage", "collection", "special", and "private-use".
       This field only appears in records whose 'Type' field-body is
       either 'language' or 'extlang'.  When this field is omitted,
       the language is an individual language.
 o  'Comments'
  • Comments's field-body contains additional information about the

subtag, as deemed appropriate for understanding the registry

       and implementing language tags using the subtag or tag.
 Future versions of this document might add additional fields to the
 registry; implementations SHOULD ignore fields found in the registry
 that are not defined in this document.

Phillips & Davis Best Current Practice [Page 25] RFC 5646 Language Tags September 2009

3.1.3. Type Field

 The field 'Type' contains the string identifying the record type in
 which it appears.  Values for the 'Type' field-body are: "language"
 (Section 2.2.1); "extlang" (Section 2.2.2); "script" (Section 2.2.3);
 "region" (Section 2.2.4); "variant" (Section 2.2.5); "grandfathered"
 or "redundant" (Section 2.2.8).

3.1.4. Subtag and Tag Fields

 The field 'Subtag' contains the subtag defined in the record.  The
 field 'Tag' appears in records whose 'Type' is either 'grandfathered'
 or 'redundant' and contains a tag registered under [RFC3066].
 The 'Subtag' field-body MUST follow the casing conventions described
 in Section 2.1.1.  All subtags use lowercase letters in the field-
 body, with two exceptions:
    Subtags whose 'Type' field is 'script' (in other words, subtags
    defined by ISO 15924) MUST use titlecase.
    Subtags whose 'Type' field is 'region' (in other words, the non-
    numeric region subtags defined by ISO 3166-1) MUST use all
    uppercase.
 The 'Tag' field-body MUST be formatted according to the rules
 described in Section 2.1.1.

3.1.5. Description Field

 The field 'Description' contains a description of the tag or subtag
 in the record.  The 'Description' field MAY appear more than once per
 record.  The 'Description' field MAY include the full range of
 Unicode characters.  At least one of the 'Description' fields MUST be
 written or transcribed into the Latin script; additional
 'Description' fields MAY be in any script or language.
 The 'Description' field is used for identification purposes.
 Descriptions SHOULD contain all and only that information necessary
 to distinguish one subtag from others with which it might be
 confused.  They are not intended to provide general background
 information or to provide all possible alternate names or
 designations.  'Description' fields don't necessarily represent the
 actual native name of the item in the record, nor are any of the
 descriptions guaranteed to be in any particular language (such as
 English or French, for example).

Phillips & Davis Best Current Practice [Page 26] RFC 5646 Language Tags September 2009

 Descriptions in the registry that correspond to ISO 639, ISO 15924,
 ISO 3166-1, or UN M.49 codes are intended only to indicate the
 meaning of that identifier as defined in the source standard at the
 time it was added to the registry or as subsequently modified, within
 the bounds of the stability rules (Section 3.4), via subsequent
 registration.  The 'Description' does not replace the content of the
 source standard itself.  'Description' fields are not intended to be
 the localized English names for the subtags.  Localization or
 translation of language tag and subtag descriptions is out of scope
 of this document.
 For subtags taken from a source standard (such as ISO 639 or ISO
 15924), the 'Description' fields in the record are also initially
 taken from that source standard.  Multiple descriptions in the source
 standard are split into separate 'Description' fields.  The source
 standard's descriptions MAY be edited or modified, either prior to
 insertion or via the registration process, and additional or
 extraneous descriptions omitted or removed.  Each 'Description' field
 MUST be unique within the record in which it appears, and formatting
 variations of the same description SHOULD NOT occur in that specific
 record.  For example, while the ISO 639-1 code 'fy' has both the
 description "Western Frisian" and the description "Frisian, Western"
 in that standard, only one of these descriptions appears in the
 registry.
 To help ensure that users do not become confused about which subtag
 to use, 'Description' fields assigned to a record of any specific
 type ('language', 'extlang', 'script', and so on) MUST be unique
 within that given record type with the following exception: if a
 particular 'Description' field occurs in multiple records of a given
 type, then at most one of the records can omit the 'Deprecated'
 field.  All deprecated records that share a 'Description' MUST have
 the same 'Preferred-Value', and all non-deprecated records MUST be
 that 'Preferred-Value'.  This means that two records of the same type
 that share a 'Description' are also semantically equivalent and no
 more than one record with a given 'Description' is preferred for that
 meaning.
 For example, consider the 'language' subtags 'zza' (Zaza) and 'diq'
 (Dimli).  It so happens that 'zza' is a macrolanguage enclosing 'diq'
 and thus also has a description in ISO 639-3 of "Dimli".  This
 description was edited to read "Dimli (macrolanguage)" in the
 registry record for 'zza' to prevent a collision.
 By contrast, the subtags 'he' and 'iw' share a 'Description' value of
 "Hebrew"; this is permitted because 'iw' is deprecated and its
 'Preferred-Value' is 'he'.

Phillips & Davis Best Current Practice [Page 27] RFC 5646 Language Tags September 2009

 For fields of type 'language', the first 'Description' field
 appearing in the registry corresponds whenever possible to the
 Reference Name assigned by ISO 639-3.  This helps facilitate cross-
 referencing between ISO 639 and the registry.
 When creating or updating a record due to the action of one of the
 source standards, the Language Subtag Reviewer MAY edit descriptions
 to correct irregularities in formatting (such as misspellings,
 inappropriate apostrophes or other punctuation, or excessive or
 missing spaces) prior to submitting the proposed record to the
 ietf-languages@iana.org list for consideration.

3.1.6. Deprecated Field

 The field 'Deprecated' contains the date the record was deprecated
 and MAY be added, changed, or removed from any record via the
 maintenance process described in Section 3.3 or via the registration
 process described in Section 3.5.  Usually, the addition of a
 'Deprecated' field is due to the action of one of the standards
 bodies, such as ISO 3166, withdrawing a code.  Although valid in
 language tags, subtags and tags with a 'Deprecated' field are
 deprecated, and validating processors SHOULD NOT generate these
 subtags.  Note that a record that contains a 'Deprecated' field and
 no corresponding 'Preferred-Value' field has no replacement mapping.
 In some historical cases, it might not have been possible to
 reconstruct the original deprecation date.  For these cases, an
 approximate date appears in the registry.  Some subtags and some
 grandfathered or redundant tags were deprecated before the initial
 creation of the registry.  The exact rules for this appear in Section
 2 of [RFC4645].  Note that these records have a 'Deprecated' field
 with an earlier date then the corresponding 'Added' field!

3.1.7. Preferred-Value Field

 The field 'Preferred-Value' contains a mapping between the record in
 which it appears and another tag or subtag (depending on the record's
 'Type').  The value in this field is used for canonicalization (see
 Section 4.5).  In cases where the subtag or tag also has a
 'Deprecated' field, then the 'Preferred-Value' is RECOMMENDED as the
 best choice to represent the value of this record when selecting a
 language tag.
 Records containing a 'Preferred-Value' fall into one of these four
 groups:

Phillips & Davis Best Current Practice [Page 28] RFC 5646 Language Tags September 2009

 1.  ISO 639 language codes that were later withdrawn in favor of
     other codes.  These values are mostly a historical curiosity.
     The 'he'/'iw' pairing above is an example of this.
 2.  Subtags (with types other than language or extlang) taken from
     codes or values that have been withdrawn in favor of a new code.
     In particular, this applies to region subtags taken from ISO
     3166-1, because sometimes a country will change its name or
     administration in such a way that warrants a new region code.  In
     some cases, countries have reverted to an older name, which might
     already be encoded.  For example, the subtag 'ZR' (Zaire) was
     replaced by the subtag 'CD' (Democratic Republic of the Congo)
     when that country's name was changed.
 3.  Tags or subtags that have become obsolete because the values they
     represent were later encoded.  Many of the grandfathered or
     redundant tags were later encoded by ISO 639, for example, and
     fall into this grouping.  For example, "i-klingon" was deprecated
     when the subtag 'tlh' was added.  The record for "i-klingon" has
     a 'Preferred-Value' of 'tlh'.
 4.  Extended language subtags always have a mapping to their
     identical primary language subtag.  For example, the extended
     language subtag 'yue' (Cantonese) can be used to form the tag
     "zh-yue".  It has a 'Preferred-Value' mapping to the primary
     language subtag 'yue', meaning that a tag such as
     "zh-yue-Hant-HK" can be canonicalized to "yue-Hant-HK".
 Records other than those of type 'extlang' that contain a 'Preferred-
 Value' field MUST also have a 'Deprecated' field.  This field
 contains the date on which the tag or subtag was deprecated in favor
 of the preferred value.
 For records of type 'extlang', the 'Preferred-Value' field appears
 without a corresponding 'Deprecated' field.  An implementation MAY
 ignore these preferred value mappings, although if it ignores the
 mapping, it SHOULD do so consistently.  It SHOULD also treat the
 'Preferred-Value' as equivalent to the mapped item.  For example, the
 tags "zh-yue-Hant-HK" and "yue-Hant-HK" are semantically equivalent
 and ought to be treated as if they were the same tag.
 Occasionally, the deprecated code is preferred in certain contexts.
 For example, both "iw" and "he" can be used in the Java programming
 language, but "he" is converted on input to "iw", which is thus the
 canonical form in Java.

Phillips & Davis Best Current Practice [Page 29] RFC 5646 Language Tags September 2009

 'Preferred-Value' mappings in records of type 'region' sometimes do
 not represent exactly the same meaning as the original value.  There
 are many reasons for a country code to be changed, and the effect
 this has on the formation of language tags will depend on the nature
 of the change in question.  For example, the region subtag 'YD'
 (Democratic Yemen) was deprecated in favor of the subtag 'YE' (Yemen)
 when those two countries unified in 1990.
 A 'Preferred-Value' MAY be added to, changed, or removed from records
 according to the rules in Section 3.3.  Addition, modification, or
 removal of a 'Preferred-Value' field in a record does not imply that
 content using the affected subtag needs to be retagged.
 The 'Preferred-Value' fields in records of type "grandfathered" and
 "redundant" each contain an "extended language range" [RFC4647] that
 is strongly RECOMMENDED for use in place of the record's value.  In
 many cases, these mappings were created via deprecation of the tags
 during the period before [RFC4646] was adopted.  For example, the tag
 "no-nyn" was deprecated in favor of the ISO 639-1-defined language
 code 'nn'.
 The 'Preferred-Value' field in subtag records of type "extlang" also
 contains an "extended language range".  This allows the subtag to be
 deprecated in favor of either a single primary language subtag or a
 new language-extlang sequence.
 Usually, the addition, removal, or change of a 'Preferred-Value'
 field for a subtag is done to reflect changes in one of the source
 standards.  For example, if an ISO 3166-1 region code is deprecated
 in favor of another code, that SHOULD result in the addition of a
 'Preferred-Value' field.
 Changes to one subtag can affect other subtags as well: when
 proposing changes to the registry, the Language Subtag Reviewer MUST
 review the registry for such effects and propose the necessary
 changes using the process in Section 3.5, although anyone MAY request
 such changes.  For example:
    Suppose that subtag 'XX' has a 'Preferred-Value' of 'YY'.  If 'YY'
    later changes to have a 'Preferred-Value' of 'ZZ', then the
    'Preferred-Value' for 'XX' MUST also change to be 'ZZ'.
    Suppose that a registered language subtag 'dialect' represents a
    language not yet available in any part of ISO 639.  The later
    addition of a corresponding language code in ISO 639 SHOULD result
    in the addition of a 'Preferred-Value' for 'dialect'.

Phillips & Davis Best Current Practice [Page 30] RFC 5646 Language Tags September 2009

3.1.8. Prefix Field

 The field 'Prefix' contains a valid language tag that is RECOMMENDED
 as one possible prefix to this record's subtag, perhaps with other
 subtags.  That is, when including an extended language or a variant
 subtag that has at least one 'Prefix' in a language tag, the
 resulting tag SHOULD match at least one of the subtag's 'Prefix'
 fields using the "Extended Filtering" algorithm (see [RFC4647]), and
 each of the subtags in that 'Prefix' SHOULD appear before the subtag
 itself.
 The 'Prefix' field MUST appear exactly once in a record of type
 'extlang'.  The 'Prefix' field MAY appear multiple times (or not at
 all) in records of type 'variant'.  Additional fields of this type
 MAY be added to a 'variant' record via the registration process,
 provided the 'variant' record already has at least one 'Prefix'
 field.
 Each 'Prefix' field indicates a particular sequence of subtags that
 form a meaningful tag with this subtag.  For example, the extended
 language subtag 'cmn' (Mandarin Chinese) only makes sense with its
 prefix 'zh' (Chinese).  Similarly, 'rozaj' (Resian, a dialect of
 Slovenian) would be appropriate when used with its prefix 'sl'
 (Slovenian), while tags such as "is-1994" are not appropriate (and
 probably not meaningful).  Although the 'Prefix' for 'rozaj' is "sl",
 other subtags might appear between them.  For example, the tag "sl-
 IT-rozaj" (Slovenian, Italy, Resian) matches the 'Prefix' "sl".
 The 'Prefix' also indicates when variant subtags make sense when used
 together (many that otherwise share a 'Prefix' are mutually
 exclusive) and what the relative ordering of variants is supposed to
 be.  For example, the variant '1994' (Standardized Resian
 orthography) has several 'Prefix' fields in the registry ("sl-rozaj",
 "sl-rozaj-biske", "sl-rozaj-njiva", "sl-rozaj-osojs", and "sl-rozaj-
 solba").  This indicates not only that '1994' is appropriate to use
 with each of these five Resian variant subtags ('rozaj', 'biske',
 'njiva', 'osojs', and 'solba'), but also that it SHOULD appear
 following any of these variants in a tag.  Thus, the language tag
 ought to take the form "sl-rozaj-biske-1994", rather than "sl-1994-
 rozaj-biske" or "sl-rozaj-1994-biske".
 If a record includes no 'Prefix' field, a 'Prefix' field MUST NOT be
 added to the record at a later date.  Otherwise, changes (additions,
 deletions, or modifications) to the set of 'Prefix' fields MAY be
 registered, as long as they strictly widen the range of language tags
 that are recommended.  For example, a 'Prefix' with the value "be-
 Latn" (Belarusian, Latin script) could be replaced by the value "be"
 (Belarusian) but not by the value "ru-Latn" (Russian, Latin script)

Phillips & Davis Best Current Practice [Page 31] RFC 5646 Language Tags September 2009

 or the value "be-Latn-BY" (Belarusian, Latin script, Belarus), since
 these latter either change or narrow the range of suggested tags.
 The field-body of the 'Prefix' field MUST NOT conflict with any
 'Prefix' already registered for a given record.  Such a conflict
 would occur when no valid tag could be constructed that would contain
 the prefix, such as when two subtags each have a 'Prefix' that
 contains the other subtag.  For example, suppose that the subtag
 'avariant' has the prefix "es-bvariant".  Then the subtag 'bvariant'
 cannot be assigned the prefix 'avariant', for that would require a
 tag of the form "es-avariant-bvariant-avariant", which would not be
 valid.

3.1.9. Suppress-Script Field

 The field 'Suppress-Script' contains a script subtag (whose record
 appears in the registry).  The field 'Suppress-Script' MUST appear
 only in records whose 'Type' field-body is either 'language' or
 'extlang'.  This field MUST NOT appear more than one time in a
 record.
 This field indicates a script used to write the overwhelming majority
 of documents for the given language.  The subtag for such a script
 therefore adds no distinguishing information to a language tag and
 thus SHOULD NOT be used for most documents in that language.
 Omitting the script subtag indicated by this field helps ensure
 greater compatibility between the language tags generated according
 to the rules in this document and language tags and tag processors or
 consumers based on RFC 3066.  For example, virtually all Icelandic
 documents are written in the Latin script, making the subtag 'Latn'
 redundant in the tag "is-Latn".
 Many language subtag records do not have a 'Suppress-Script' field.
 The lack of a 'Suppress-Script' might indicate that the language is
 customarily written in more than one script or that the language is
 not customarily written at all.  It might also mean that sufficient
 information was not available when the record was created and thus
 remains a candidate for future registration.

3.1.10. Macrolanguage Field

 The field 'Macrolanguage' contains a primary language subtag (whose
 record appears in the registry).  This field indicates a language
 that encompasses this subtag's language according to assignments made
 by ISO 639-3.
 ISO 639-3 labels some languages in the registry as "macrolanguages".
 ISO 639-3 defines the term "macrolanguage" to mean "clusters of

Phillips & Davis Best Current Practice [Page 32] RFC 5646 Language Tags September 2009

 closely-related language varieties that [...] can be considered
 distinct individual languages, yet in certain usage contexts a single
 language identity for all is needed".  These correspond to codes
 registered in ISO 639-2 as individual languages that were found to
 correspond to more than one language in ISO 639-3.
 A language contained within a macrolanguage is called an "encompassed
 language".  The record for each encompassed language contains a
 'Macrolanguage' field in the registry; the macrolanguages themselves
 are not specially marked.  Note that some encompassed languages have
 ISO 639-1 or ISO 639-2 codes.
 The 'Macrolanguage' field can only occur in records of type
 'language' or 'extlang'.  Only values assigned by ISO 639-3 will be
 considered for inclusion.  'Macrolanguage' fields MAY be added or
 removed via the normal registration process whenever ISO 639-3
 defines new values or withdraws old values.  Macrolanguages are
 informational, and MAY be removed or changed if ISO 639-3 changes the
 values.  For more information on the use of this field and choosing
 between macrolanguage and encompassed language subtags, see
 Section 4.1.1.
 For example, the language subtags 'nb' (Norwegian Bokmal) and 'nn'
 (Norwegian Nynorsk) each have a 'Macrolanguage' field with a value of
 'no' (Norwegian).  For more information, see Section 4.1.

3.1.11. Scope Field

 The field 'Scope' contains classification information about a primary
 or extended language subtag derived from ISO 639.  Most languages
 have a scope of 'individual', which means that the language is not a
 macrolanguage, collection, special code, or private use.  That is, it
 is what one would normally consider to be 'a language'.  Any primary
 or extended language subtag that has no 'Scope' field is an
 individual language.
 'Scope' information can sometimes be helpful in selecting language
 tags, since it indicates the purpose or "scope" of the code
 assignment within ISO 639.  The available values are:
 o  'macrolanguage' - Indicates a macrolanguage as defined by ISO
    639-3 (see Section 3.1.10).  A macrolanguage is a cluster of
    closely related languages that are sometimes considered to be a
    single language.
 o  'collection' - Indicates a subtag that represents a collection of
    languages, typically related by some type of historical,
    geographical, or linguistic association.  Unlike a macrolanguage,

Phillips & Davis Best Current Practice [Page 33] RFC 5646 Language Tags September 2009

    a collection can contain languages that are only loosely related
    and a collection cannot be used interchangeably with languages
    that belong to it.
 o  'special' - Indicates a special language code.  These are subtags
    used for identifying linguistic attributes not particularly
    associated with a concrete language.  These include codes for when
    the language is undetermined or for non-linguistic content.
 o  'private-use' - Indicates a code reserved for private use in the
    underlying standard.  Subtags with this scope can be used to
    indicate a primary language for which no ISO 639 or registered
    assignment exists.
 The 'Scope' field MAY appear in records of type 'language' or
 'extlang'.  Note that many of the prefixes for extended language
 subtags will have a 'Scope' of 'macrolanguage' (although some will
 not) and that many languages that have a 'Scope' of 'macrolanguage'
 will have extended language subtags associated with them.
 The 'Scope' field MAY be added, modified, or removed via the
 registration process, provided the change mirrors changes made by ISO
 639 to the assignment's classification.  Such a change is expected to
 be rare.
 For example, the primary language subtag 'zh' (Chinese) has a 'Scope'
 of 'macrolanguage', while its enclosed language 'nan' (Min Nan
 Chinese) has a 'Scope' of 'individual'.  The special value 'und'
 (Undetermined) has a 'Scope' of 'special'.  The ISO 639-5 collection
 'gem' (Germanic languages) has a 'Scope' of 'collection'.

3.1.12. Comments Field

 The field 'Comments' contains additional information about the record
 and MAY appear more than once per record.  The field-body MAY include
 the full range of Unicode characters and is not restricted to any
 particular script.  This field MAY be inserted or changed via the
 registration process, and no guarantee of stability is provided.
 The content of this field is not restricted, except by the need to
 register the information, the suitability of the request, and by
 reasonable practical size limitations.  The primary reason for the
 'Comments' field is subtag identification -- to help distinguish the
 subtag from others with which it might be confused as an aid to
 usage.  Large amounts of information about the use, history, or
 general background of a subtag are frowned upon, as these generally
 belong in a registration request rather than in the registry.

Phillips & Davis Best Current Practice [Page 34] RFC 5646 Language Tags September 2009

3.2. Language Subtag Reviewer

 The Language Subtag Reviewer moderates the ietf-languages@iana.org
 mailing list, responds to requests for registration, and performs the
 other registry maintenance duties described in Section 3.3.  Only the
 Language Subtag Reviewer is permitted to request IANA to change,
 update, or add records to the Language Subtag Registry.  The Language
 Subtag Reviewer MAY delegate list moderation and other clerical
 duties as needed.
 The Language Subtag Reviewer is appointed by the IESG for an
 indefinite term, subject to removal or replacement at the IESG's
 discretion.  The IESG will solicit nominees for the position (upon
 adoption of this document or upon a vacancy) and then solicit
 feedback on the nominees' qualifications.  Qualified candidates
 should be familiar with BCP 47 and its requirements; be willing to
 fairly, responsively, and judiciously administer the registration
 process; and be suitably informed about the issues of language
 identification so that the reviewer can assess the claims and draw
 upon the contributions of language experts and subtag requesters.
 The subsequent performance or decisions of the Language Subtag
 Reviewer MAY be appealed to the IESG under the same rules as other
 IETF decisions (see [RFC2026]).  The IESG can reverse or overturn the
 decisions of the Language Subtag Reviewer, provide guidance, or take
 other appropriate actions.

3.3. Maintenance of the Registry

 Maintenance of the registry requires that, as codes are assigned or
 withdrawn by ISO 639, ISO 15924, ISO 3166, and UN M.49, the Language
 Subtag Reviewer MUST evaluate each change and determine the
 appropriate course of action according to the rules in this document.
 Such updates follow the registration process described in
 Section 3.5.  Usually, the Language Subtag Reviewer will start the
 process for the new or updated record by filling in the registration
 form and submitting it.  If a change to one of these standards takes
 place and the Language Subtag Reviewer does not do this in a timely
 manner, then any interested party MAY submit the form.  Thereafter,
 the registration process continues normally.
 Note that some registrations affect other subtags--perhaps more than
 one--as when a region subtag is being deprecated in favor of a new
 value.  The Language Subtag Reviewer is responsible for ensuring that
 any such changes are properly registered, with each change requiring
 its own registration form.

Phillips & Davis Best Current Practice [Page 35] RFC 5646 Language Tags September 2009

 The Language Subtag Reviewer MUST ensure that new subtags meet the
 requirements elsewhere in this document (and most especially in
 Section 3.4) or submit an appropriate registration form for an
 alternate subtag as described in that section.  Each individual
 subtag affected by a change MUST be sent to the
 ietf-languages@iana.org list with its own registration form and in a
 separate message.

3.4. Stability of IANA Registry Entries

 The stability of entries and their meaning in the registry is
 critical to the long-term stability of language tags.  The rules in
 this section guarantee that a specific language tag's meaning is
 stable over time and will not change.
 These rules specifically deal with how changes to codes (including
 withdrawal and deprecation of codes) maintained by ISO 639, ISO
 15924, ISO 3166, and UN M.49 are reflected in the IANA Language
 Subtag Registry.  Assignments to the IANA Language Subtag Registry
 MUST follow the following stability rules:
 1.   Values in the fields 'Type', 'Subtag', 'Tag', and 'Added' MUST
      NOT be changed and are guaranteed to be stable over time.
 2.   Values in the fields 'Preferred-Value' and 'Deprecated' MAY be
      added, altered, or removed via the registration process.  These
      changes SHOULD be limited to changes necessary to mirror changes
      in one of the underlying standards (ISO 639, ISO 15924, ISO
      3166-1, or UN M.49) and typically alteration or removal of a
      'Preferred-Value' is limited specifically to region codes.
 3.   Values in the 'Description' field MUST NOT be changed in a way
      that would invalidate any existing tags.  The description MAY be
      broadened somewhat in scope, changed to add information, or
      adapted to the most common modern usage.  For example, countries
      occasionally change their names; a historical example of this is
      "Upper Volta" changing to "Burkina Faso".
 4.   Values in the field 'Prefix' MAY be added to existing records of
      type 'variant' via the registration process, provided the
      'variant' already has at least one 'Prefix'.  A 'Prefix' field
      SHALL NOT be registered for any 'variant' that has no existing
      'Prefix' field.  If a prefix is added to a variant record,
      'Comment' fields MAY be used to explain different usages with
      the various prefixes.

Phillips & Davis Best Current Practice [Page 36] RFC 5646 Language Tags September 2009

 5.   Values in the field 'Prefix' in records of type 'variant' MAY
      also be modified, so long as the modifications broaden the set
      of prefixes.  That is, a prefix MAY be replaced by one of its
      own prefixes.  For example, the prefix "en-US" could be replaced
      by "en", but not by the prefixes "en-Latn", "fr", or "en-US-
      boont".  If one of those prefix values were needed, it would
      have to be separately registered.
 6.   Values in the field 'Prefix' in records of type 'extlang' MUST
      NOT be added, modified, or removed.
 7.   The field 'Prefix' MUST NOT be removed from any record in which
      it appears.  This field SHOULD be included in the initial
      registration of any records of type 'variant' and MUST be
      included in any records of type 'extlang'.
 8.   The field 'Comments' MAY be added, changed, modified, or removed
      via the registration process or any of the processes or
      considerations described in this section.
 9.   The field 'Suppress-Script' MAY be added or removed via the
      registration process.
 10.  The field 'Macrolanguage' MAY be added or removed via the
      registration process, but only in response to changes made by
      ISO 639.  The 'Macrolanguage' field appears whenever a language
      has a corresponding macrolanguage in ISO 639.  That is, the
      'Macrolanguage' fields in the registry exactly match those of
      ISO 639.  No other macrolanguage mappings will be considered for
      registration.
 11.  The field 'Scope' MAY be added or removed from a primary or
      extended language subtag after initial registration, and it MAY
      be modified in order to match any changes made by ISO 639.
      Changes to the 'Scope' field MUST mirror changes made by ISO
      639.  Note that primary or extended language subtags whose
      records do not contain a 'Scope' field (that is, most of them)
      are individual languages as described in Section 3.1.11.
 12.  Primary and extended language subtags (other than independently
      registered values created using the registration process) are
      created according to the assignments of the various parts of ISO
      639, as follows:
      A.  Codes assigned by ISO 639-1 that do not conflict with
          existing two-letter primary language subtags and that have
          no corresponding three-letter primary defined in the
          registry are entered into the IANA registry as new records

Phillips & Davis Best Current Practice [Page 37] RFC 5646 Language Tags September 2009

          of type 'language'.  Note that languages given an ISO 639-1
          code cannot be given extended language subtags, even if
          encompassed by a macrolanguage.
      B.  Codes assigned by ISO 639-3 or ISO 639-5 that do not
          conflict with existing three-letter primary language subtags
          and that do not have ISO 639-1 codes assigned (or expected
          to be assigned) are entered into the IANA registry as new
          records of type 'language'.  Note that these two standards
          now comprise a superset of ISO 639-2 codes.  Codes that have
          a defined 'macrolanguage' mapping at the time of their
          registration MUST contain a 'Macrolanguage' field.
      C.  Codes assigned by ISO 639-3 MAY also be considered for an
          extended language subtag registration.  Note that they MUST
          be assigned a primary language subtag record of type
          'language' even when an 'extlang' record is proposed.  When
          considering extended language subtag assignment, these
          criteria apply:
          1.  If a language has a macrolanguage mapping, and that
              macrolanguage has other encompassed languages that are
              assigned extended language subtags, then the new
              language SHOULD have an 'extlang' record assigned to it
              as well.  For example, any language with a macrolanguage
              of 'zh' or 'ar' would be assigned an 'extlang' record.
          2.  'Extlang' records SHOULD NOT be created for languages if
              other languages encompassed by the macrolanguage do not
              also include 'extlang' records.  For example, if a new
              Serbo-Croatian ('sh') language were registered, it would
              not get an extlang record because other languages
              encompassed, such as Serbian ('sr'), do not include one
              in the registry.
          3.  Sign languages SHOULD have an 'extlang' record with a
              'Prefix' of 'sgn'.
          4.  'Extlang' records MUST NOT be created for items already
              in the registry.  Extended language subtags will only be
              considered at the time of initial registration.
          5.  Extended language subtag records MUST include the fields
              'Prefix' and 'Preferred-Value' with field values
              assigned as described in Section 2.2.2.
      D.  Any other codes assigned by ISO 639-2 that do not conflict
          with existing three-letter primary or extended language

Phillips & Davis Best Current Practice [Page 38] RFC 5646 Language Tags September 2009

          subtags and that do not have ISO 639-1 two-letter codes
          assigned are entered into the IANA registry as new records
          of type 'language'.  This type of registration is not
          supposed to occur in the future.
 13.  Codes assigned by ISO 15924 and ISO 3166-1 that do not conflict
      with existing subtags of the associated type and whose meaning
      is not the same as an existing subtag of the same type are
      entered into the IANA registry as new records.
 14.  Codes assigned by ISO 639, ISO 15924, or ISO 3166-1 that are
      withdrawn by their respective maintenance or registration
      authority remain valid in language tags.  A 'Deprecated' field
      containing the date of withdrawal MUST be added to the record.
      If a new record of the same type is added that represents a
      replacement value, then a 'Preferred-Value' field MAY also be
      added.  The registration process MAY be used to add comments
      about the withdrawal of the code by the respective standard.
         For example: the region code 'TL' was assigned to the country
         'Timor-Leste', replacing the code 'TP' (which was assigned to
         'East Timor' when it was under administration by Portugal).
         The subtag 'TP' remains valid in language tags, but its
         record contains the 'Preferred-Value' of 'TL' and its field
         'Deprecated' contains the date the new code was assigned
         ('2004-07-06').
 15.  Codes assigned by ISO 639, ISO 15924, or ISO 3166-1 that
      conflict with existing subtags of the associated type, including
      subtags that are deprecated, MUST NOT be entered into the
      registry.  The following additional considerations apply to
      subtag values that are reassigned:
      A.  For ISO 639 codes, if the newly assigned code's meaning is
          not represented by a subtag in the IANA registry, the
          Language Subtag Reviewer, as described in Section 3.5, SHALL
          prepare a proposal for entering in the IANA registry, as
          soon as practical, a registered language subtag as an
          alternate value for the new code.  The form of the
          registered language subtag will be at the discretion of the
          Language Subtag Reviewer and MUST conform to other
          restrictions on language subtags in this document.
      B.  For all subtags whose meaning is derived from an external
          standard (that is, by ISO 639, ISO 15924, ISO 3166-1, or UN
          M.49), if a new meaning is assigned to an existing code and
          the new meaning broadens the meaning of that code, then the
          meaning for the associated subtag MAY be changed to match.

Phillips & Davis Best Current Practice [Page 39] RFC 5646 Language Tags September 2009

          The meaning of a subtag MUST NOT be narrowed, however, as
          this can result in an unknown proportion of the existing
          uses of a subtag becoming invalid.  Note: the ISO 639
          registration authority (RA) has adopted a similar stability
          policy.
      C.  For ISO 15924 codes, if the newly assigned code's meaning is
          not represented by a subtag in the IANA registry, the
          Language Subtag Reviewer, as described in Section 3.5, SHALL
          prepare a proposal for entering in the IANA registry, as
          soon as practical, a registered variant subtag as an
          alternate value for the new code.  The form of the
          registered variant subtag will be at the discretion of the
          Language Subtag Reviewer and MUST conform to other
          restrictions on variant subtags in this document.
      D.  For ISO 3166-1 codes, if the newly assigned code's meaning
          is associated with the same UN M.49 code as another 'region'
          subtag, then the existing region subtag remains as the
          preferred value for that region and no new entry is created.
          A comment MAY be added to the existing region subtag
          indicating the relationship to the new ISO 3166-1 code.
      E.  For ISO 3166-1 codes, if the newly assigned code's meaning
          is associated with a UN M.49 code that is not represented by
          an existing region subtag, then the Language Subtag
          Reviewer, as described in Section 3.5, SHALL prepare a
          proposal for entering the appropriate UN M.49 country code
          as an entry in the IANA registry.
      F.  For ISO 3166-1 codes, if there is no associated UN numeric
          code, then the Language Subtag Reviewer SHALL petition the
          UN to create one.  If there is no response from the UN
          within 90 days of the request being sent, the Language
          Subtag Reviewer SHALL prepare a proposal for entering in the
          IANA registry, as soon as practical, a registered variant
          subtag as an alternate value for the new code.  The form of
          the registered variant subtag will be at the discretion of
          the Language Subtag Reviewer and MUST conform to other
          restrictions on variant subtags in this document.  This
          situation is very unlikely to ever occur.
 16.  UN M.49 has codes for both "countries and areas" (such as '276'
      for Germany) and "geographical regions and sub-regions" (such as
      '150' for Europe).  UN M.49 country or area codes for which
      there is no corresponding ISO 3166-1 code MUST NOT be
      registered, except as a surrogate for an ISO 3166-1 code that is
      blocked from registration by an existing subtag.

Phillips & Davis Best Current Practice [Page 40] RFC 5646 Language Tags September 2009

      If such a code becomes necessary, then the maintenance agency
      for ISO 3166-1 SHALL first be petitioned to assign a code to the
      region.  If the petition for a code assignment by ISO 3166-1 is
      refused or not acted on in a timely manner, the registration
      process described in Section 3.5 can then be used to register
      the corresponding UN M.49 code.  This way, UN M.49 codes remain
      available as the value of last resort in cases where ISO 3166-1
      reassigns a deprecated value in the registry.
 17.  The redundant and grandfathered entries together form the
      complete list of tags registered under [RFC3066].  The redundant
      tags are those previously registered tags that can now be formed
      using the subtags defined in the registry.  The grandfathered
      entries include those that can never be legal because they are
      'irregular' (that is, they do not match the 'langtag' production
      in Figure 1), are limited by rule (subtags such as 'nyn' and
      'min' look like the extlang production, but cannot be registered
      as extended language subtags), or their subtags are
      inappropriate for registration.  All of the grandfathered tags
      are listed in either the 'regular' or the 'irregular'
      productions in the ABNF.  Under [RFC4646] it was possible for
      grandfathered tags to become redundant.  However, all of the
      tags for which this was possible became redundant before this
      document was produced.  So the set of redundant and
      grandfathered tags is now permanent and immutable: new entries
      of either type MUST NOT be added and existing entries MUST NOT
      be removed.  The decision-making process about which tags were
      initially grandfathered and which were made redundant is
      described in [RFC4645].
      Many of the grandfathered tags are deprecated -- indeed, they
      were deprecated even before [RFC4646].  For example, the tag
      "art-lojban" was deprecated in favor of the primary language
      subtag 'jbo'.  These tags could have been made 'redundant' by
      registering some of their subtags as 'variants'.  The 'variant-
      like' subtags in the grandfathered registrations SHALL NOT be
      registered in the future, even with a similar or identical
      meaning.

3.5. Registration Procedure for Subtags

 The procedure given here MUST be used by anyone who wants to use a
 subtag not currently in the IANA Language Subtag Registry or who
 wishes to add, modify, update, or remove information in existing
 records as permitted by this document.
 Only subtags of type 'language' and 'variant' will be considered for
 independent registration of new subtags.  Subtags needed for

Phillips & Davis Best Current Practice [Page 41] RFC 5646 Language Tags September 2009

 stability and subtags necessary to keep the registry synchronized
 with ISO 639, ISO 15924, ISO 3166, and UN M.49 within the limits
 defined by this document also use this process, as described in
 Section 3.3 and subject to stability provisions as described in
 Section 3.4.
 Registration requests are accepted relating to information in the
 'Comments', 'Deprecated', 'Description', 'Prefix', 'Preferred-Value',
 'Macrolanguage', or 'Suppress-Script' fields in a subtag's record as
 described in Section 3.4.  Changes to all other fields in the IANA
 registry are NOT permitted.
 Registering a new subtag or requesting modifications to an existing
 tag or subtag starts with the requester filling out the registration
 form reproduced below.  Note that each response is not limited in
 size so that the request can adequately describe the registration.
 The fields in the "Record Requested" section need to follow the
 requirements in Section 3.1 before the record will be approved.
 LANGUAGE SUBTAG REGISTRATION FORM
 1. Name of requester:
 2. E-mail address of requester:
 3. Record Requested:
    Type:
    Subtag:
    Description:
    Prefix:
    Preferred-Value:
    Deprecated:
    Suppress-Script:
    Macrolanguage:
    Comments:
 4. Intended meaning of the subtag:
 5. Reference to published description
    of the language (book or article):
 6. Any other relevant information:
            Figure 5: The Language Subtag Registration Form
 Examples of completed registration forms can be found in Appendix B.
 A complete list of approved registration forms is online through
 http://www.iana.org; readers should note that the Language Tag
 Registry is now obsolete and should instead look for the Language
 Subtag Registry.

Phillips & Davis Best Current Practice [Page 42] RFC 5646 Language Tags September 2009

 The subtag registration form MUST be sent to
 <ietf-languages@iana.org>.  Registration requests receive a two-week
 review period before being approved and submitted to IANA for
 inclusion in the registry.  If modifications are made to the request
 during the course of the registration process (such as corrections to
 meet the requirements in Section 3.1 or to make the 'Description'
 fields unique for the given record type), the modified form MUST also
 be sent to <ietf-languages@iana.org> at least one week prior to
 submission to IANA.
 The ietf-languages list is an open list and can be joined by sending
 a request to <ietf-languages-request@iana.org>.  The list can be
 hosted by IANA or any third party at the request of IESG.
 Before forwarding any registration to IANA, the Language Subtag
 Reviewer MUST ensure that all requirements in this document are met.
 This includes ensuring that values in the 'Subtag' field match case
 according to the description in Section 3.1.4 and that 'Description'
 fields are unique for the given record type as described in
 Section 3.1.5.  The Reviewer MUST also ensure that an appropriate
 File-Date record is included in the request, to assist IANA when
 updating the registry (see Section 5.1).
 Some fields in both the registration form as well as the registry
 record itself permit the use of non-ASCII characters.  Registration
 requests SHOULD use the UTF-8 encoding for consistency and clarity.
 However, since some mail clients do not support this encoding, other
 encodings MAY be used for the registration request.  The Language
 Subtag Reviewer is responsible for ensuring that the proper Unicode
 characters appear in both the archived request form and the registry
 record.  In the case of a transcription or encoding error by IANA,
 the Language Subtag Reviewer will request that the registry be
 repaired, providing any necessary information to assist IANA.
 Extended language subtags (type 'extlang'), by definition, are always
 encompassed by another language.  All records of type 'extlang' MUST,
 therefore, contain a 'Prefix' field at the time of registration.
 This 'Prefix' field can never be altered or removed, and requests to
 do so MUST be rejected.
 Variant subtags are usually registered for use with a particular
 range of language tags, and variant subtags based on the terminology
 of the language to which they are apply are encouraged.  For example,
 the subtag 'rozaj' (Resian) is intended for use with language tags
 that start with the primary language subtag "sl" (Slovenian), since
 Resian is a dialect of Slovenian.  Thus, the subtag 'rozaj' would be
 appropriate in tags such as "sl-Latn-rozaj" or "sl-IT-rozaj".  This
 information is stored in the 'Prefix' field in the registry.  Variant

Phillips & Davis Best Current Practice [Page 43] RFC 5646 Language Tags September 2009

 registration requests SHOULD include at least one 'Prefix' field in
 the registration form.
 Requests to assign an additional record of a given type with an
 existing subtag value MUST be rejected.  For example, the variant
 subtag 'rozaj' already exists in the registry, so adding a second
 record of type 'variant' with the subtag 'rozaj' is prohibited.
 The 'Prefix' field for a given registered variant subtag exists in
 the IANA registry as a guide to usage.  Additional 'Prefix' fields
 MAY be added by filing an additional registration form.  In that
 form, the "Any other relevant information:" field MUST indicate that
 it is the addition of a prefix.
 Requests to add a 'Prefix' field to a variant subtag that imply a
 different semantic meaning SHOULD be rejected.  For example, a
 request to add the prefix "de" to the subtag '1994' so that the tag
 "de-1994" represented some German dialect or orthographic form would
 be rejected.  The '1994' subtag represents a particular Slovenian
 orthography, and the additional registration would change or blur the
 semantic meaning assigned to the subtag.  A separate subtag SHOULD be
 proposed instead.
 Requests to add a 'Prefix' to a variant subtag that has no current
 'Prefix' field MUST be rejected.  Variants are registered with no
 prefix because they are potentially useful with many or even all
 languages.  Adding one or more 'Prefix' fields would be potentially
 harmful to the use of the variant, since it dramatically reduces the
 scope of the subtag (which is not allowed under the stability rules
 (Section 3.4) as opposed to broadening the scope of the subtag, which
 is what the addition of a 'Prefix' normally does.  An example of such
 a "no-prefix" variant is the subtag 'fonipa', which represents the
 International Phonetic Alphabet, a scheme that can be used to
 transcribe many languages.
 The 'Description' fields provided in the request MUST contain at
 least one description written or transcribed into the Latin script;
 the request MAY also include additional 'Description' fields in any
 script or language.  The 'Description' field is used for
 identification purposes and doesn't necessarily represent the actual
 native name of the language or variation.  It also doesn't have to be
 in any particular language, but SHOULD be both suitable and
 sufficient to identify the item in the record.  The Language Subtag
 Reviewer will check and edit any proposed 'Description' fields so as
 to ensure uniqueness and prevent collisions with 'Description' fields
 in other records of the same type.  If this occurs in an independent
 registration request, the Language Subtag Reviewer MUST resubmit the
 record to <ietf-languages@iana.org>, treating it as a modification of

Phillips & Davis Best Current Practice [Page 44] RFC 5646 Language Tags September 2009

 a request due to discussion, as described in Section 3.5, unless the
 request's sole purpose is to introduce a duplicate 'Description'
 field, in which case the request SHALL be rejected.
 The 'Description' field is not guaranteed to be stable.  Corrections
 or clarifications of intent are examples of possible changes.
 Attempts to provide translations or transcriptions of entries in the
 registry (which, by definition, provide no new information) are
 unlikely to be approved.
 Soon after the two-week review period has passed, the Language Subtag
 Reviewer MUST take one of the following actions:
 o  Explicitly accept the request and forward the form containing the
    record to be inserted or modified to <iana@iana.org> according to
    the procedure described in Section 3.3.
 o  Explicitly reject the request because of significant objections
    raised on the list or due to problems with constraints in this
    document (which MUST be explicitly cited).
 o  Extend the review period by granting an additional two-week
    increment to permit further discussion.  After each two-week
    increment, the Language Subtag Reviewer MUST indicate on the list
    whether the registration has been accepted, rejected, or extended.
 Note that the Language Subtag Reviewer MAY raise objections on the
 list if he or she so desires.  The important thing is that the
 objection MUST be made publicly.
 Sometimes the request needs to be modified as a result of discussion
 during the review period or due to requirements in this document.
 The applicant, Language Subtag Reviewer, or others MAY submit a
 modified version of the completed registration form, which will be
 considered in lieu of the original request with the explicit approval
 of the applicant.  Such changes do not restart the two-week
 discussion period, although an application containing the final
 record submitted to IANA MUST appear on the list at least one week
 prior to the Language Subtag Reviewer forwarding the record to IANA.
 The applicant MAY modify a rejected application with more appropriate
 or additional information and submit it again; this starts a new two-
 week comment period.
 Registrations initiated due to the provisions of Section 3.3 or
 Section 3.4 SHALL NOT be rejected altogether (since they have to
 ultimately appear in the registry) and SHOULD be completed as quickly
 as possible.  The review process allows list members to comment on
 the specific information in the form and the record it contains and

Phillips & Davis Best Current Practice [Page 45] RFC 5646 Language Tags September 2009

 thus help ensure that it is correct and consistent.  The Language
 Subtag Reviewer MAY reject a specific version of the form, but MUST
 propose a suitable replacement, extending the review period as
 described above, until the form is in a format worthy of the
 reviewer's approval and meets with rough consensus of the list.
 Decisions made by the Language Subtag Reviewer MAY be appealed to the
 IESG [RFC2028] under the same rules as other IETF decisions
 [RFC2026].  This includes a decision to extend the review period or
 the failure to announce a decision in a clear and timely manner.
 The approved records appear in the Language Subtag Registry.  The
 approved registration forms are available online from
 http://www.iana.org.
 Updates or changes to existing records follow the same procedure as
 new registrations.  The Language Subtag Reviewer decides whether
 there is consensus to update the registration following the two-week
 review period; normally, objections by the original registrant will
 carry extra weight in forming such a consensus.
 Registrations are permanent and stable.  Once registered, subtags
 will not be removed from the registry and will remain a valid way in
 which to specify a specific language or variant.
 Note: The purpose of the "Reference to published description" section
 in the registration form is to aid in verifying whether a language is
 registered or to which language or language variation a particular
 subtag refers.  In most cases, reference to an authoritative grammar
 or dictionary of that language will be useful; in cases where no such
 work exists, other well-known works describing that language or in
 that language MAY be appropriate.  The Language Subtag Reviewer
 decides what constitutes "good enough" reference material.  This
 requirement is not intended to exclude particular languages or
 dialects due to the size of the speaker population or lack of a
 standardized orthography.  Minority languages will be considered
 equally on their own merits.

3.6. Possibilities for Registration

 Possibilities for registration of subtags or information about
 subtags include:
 o  Primary language subtags for languages not listed in ISO 639 that
    are not variants of any listed or registered language MAY be
    registered.  At the time this document was created, there were no
    examples of this form of subtag.  Before attempting to register a
    language subtag, there MUST be an attempt to register the language

Phillips & Davis Best Current Practice [Page 46] RFC 5646 Language Tags September 2009

    with ISO 639.  Subtags MUST NOT be registered for languages
    defined by codes that exist in ISO 639-1, ISO 639-2, or ISO 639-3;
    that are under consideration by the ISO 639 registration
    authorities; or that have never been attempted for registration
    with those authorities.  If ISO 639 has previously rejected a
    language for registration, it is reasonable to assume that there
    must be additional, very compelling evidence of need before it
    will be registered as a primary language subtag in the IANA
    registry (to the extent that it is very unlikely that any subtags
    will be registered of this type).
 o  Dialect or other divisions or variations within a language, its
    orthography, writing system, regional or historical usage,
    transliteration or other transformation, or distinguishing
    variation MAY be registered as variant subtags.  An example is the
    'rozaj' subtag (the Resian dialect of Slovenian).
 o  The addition or maintenance of fields (generally of an
    informational nature) in tag or subtag records as described in
    Section 3.1 is allowed.  Such changes are subject to the stability
    provisions in Section 3.4.  This includes 'Description',
    'Comments', 'Deprecated', and 'Preferred-Value' fields for
    obsolete or withdrawn codes, or the addition of 'Suppress-Script'
    or 'Macrolanguage' fields to primary language subtags, as well as
    other changes permitted by this document, such as the addition of
    an appropriate 'Prefix' field to a variant subtag.
 o  The addition of records and related field value changes necessary
    to reflect assignments made by ISO 639, ISO 15924, ISO 3166-1, and
    UN M.49 as described in Section 3.4 is allowed.
 Subtags proposed for registration that would cause all or part of a
 grandfathered tag to become redundant but whose meaning conflicts
 with or alters the meaning of the grandfathered tag MUST be rejected.
 This document leaves the decision on what subtags or changes to
 subtags are appropriate (or not) to the registration process
 described in Section 3.5.
 Note: Four-character primary language subtags are reserved to allow
 for the possibility of alpha4 codes in some future addition to the
 ISO 639 family of standards.
 ISO 639 defines a registration authority for additions to and changes
 in the list of languages in ISO 639.  This agency is:

Phillips & Davis Best Current Practice [Page 47] RFC 5646 Language Tags September 2009

 International Information Centre for Terminology (Infoterm)
 Aichholzgasse 6/12, AT-1120
 Wien, Austria
 Phone: +43 1 26 75 35 Ext. 312 Fax: +43 1 216 32 72
 ISO 639-2 defines a registration authority for additions to and
 changes in the list of languages in ISO 639-2.  This agency is:
 Library of Congress
 Network Development and MARC Standards Office
 Washington, DC 20540, USA
 Phone: +1 202 707 6237 Fax: +1 202 707 0115
 URL: http://www.loc.gov/standards/iso639-2
 ISO 639-3 defines a registration authority for additions to and
 changes in the list of languages in ISO 639-3.  This agency is:
 SIL International
 ISO 639-3 Registrar
 7500 W. Camp Wisdom Rd.
 Dallas, TX 75236, USA
 Phone: +1 972 708 7400, ext. 2293
 Fax: +1 972 708 7546
 Email: iso639-3@sil.org
 URL: http://www.sil.org/iso639-3
 ISO 639-5 defines a registration authority for additions to and
 changes in the list of languages in ISO 639-5.  This agency is the
 same as for ISO 639-2 and is:
 Library of Congress
 Network Development and MARC Standards Office
 Washington, DC 20540, USA
 Phone: +1 202 707 6237
 Fax: +1 202 707 0115
 URL: http://www.loc.gov/standards/iso639-5
 The maintenance agency for ISO 3166-1 (country codes) is:
 ISO 3166 Maintenance Agency
 c/o International Organization for Standardization
 Case postale 56
 CH-1211 Geneva 20, Switzerland
 Phone: +41 22 749 72 33 Fax: +41 22 749 73 49
 URL: http://www.iso.org/iso/en/prods-services/iso3166ma/index.html

Phillips & Davis Best Current Practice [Page 48] RFC 5646 Language Tags September 2009

 The registration authority for ISO 15924 (script codes) is:
 Unicode Consortium
 Box 391476
 Mountain View, CA 94039-1476, USA
 URL: http://www.unicode.org/iso15924
 The Statistics Division of the United Nations Secretariat maintains
 the Standard Country or Area Codes for Statistical Use and can be
 reached at:
 Statistical Services Branch
 Statistics Division
 United Nations, Room DC2-1620
 New York, NY 10017, USA
 Fax: +1-212-963-0623
 Email: statistics@un.org
 URL: http://unstats.un.org/unsd/methods/m49/m49alpha.htm

3.7. Extensions and the Extensions Registry

 Extension subtags are those introduced by single-character subtags
 ("singletons") other than 'x'.  They are reserved for the generation
 of identifiers that contain a language component and are compatible
 with applications that understand language tags.
 The structure and form of extensions are defined by this document so
 that implementations can be created that are forward compatible with
 applications that might be created using singletons in the future.
 In addition, defining a mechanism for maintaining singletons will
 lend stability to this document by reducing the likely need for
 future revisions or updates.
 Single-character subtags are assigned by IANA using the "IETF Review"
 policy defined by [RFC5226].  This policy requires the development of
 an RFC, which SHALL define the name, purpose, processes, and
 procedures for maintaining the subtags.  The maintaining or
 registering authority, including name, contact email, discussion list
 email, and URL location of the registry, MUST be indicated clearly in
 the RFC.  The RFC MUST specify or include each of the following:
 o  The specification MUST reference the specific version or revision
    of this document that governs its creation and MUST reference this
    section of this document.
 o  The specification and all subtags defined by the specification
    MUST follow the ABNF and other rules for the formation of tags and
    subtags as defined in this document.  In particular, it MUST

Phillips & Davis Best Current Practice [Page 49] RFC 5646 Language Tags September 2009

    specify that case is not significant and that subtags MUST NOT
    exceed eight characters in length.
 o  The specification MUST specify a canonical representation.
 o  The specification of valid subtags MUST be available over the
    Internet and at no cost.
 o  The specification MUST be in the public domain or available via a
    royalty-free license acceptable to the IETF and specified in the
    RFC.
 o  The specification MUST be versioned, and each version of the
    specification MUST be numbered, dated, and stable.
 o  The specification MUST be stable.  That is, extension subtags,
    once defined by a specification, MUST NOT be retracted or change
    in meaning in any substantial way.
 o  The specification MUST include, in a separate section, the
    registration form reproduced in this section (below) to be used in
    registering the extension upon publication as an RFC.
 o  IANA MUST be informed of changes to the contact information and
    URL for the specification.
 IANA will maintain a registry of allocated single-character
 (singleton) subtags.  This registry MUST use the record-jar format
 described by the ABNF in Section 3.1.1.  Upon publication of an
 extension as an RFC, the maintaining authority defined in the RFC
 MUST forward this registration form to <iesg@ietf.org>, who MUST
 forward the request to <iana@iana.org>.  The maintaining authority of
 the extension MUST maintain the accuracy of the record by sending an
 updated full copy of the record to <iana@iana.org> with the subject
 line "LANGUAGE TAG EXTENSION UPDATE" whenever content changes.  Only
 the 'Comments', 'Contact_Email', 'Mailing_List', and 'URL' fields MAY
 be modified in these updates.
 Failure to maintain this record, maintain the corresponding registry,
 or meet other conditions imposed by this section of this document MAY
 be appealed to the IESG [RFC2028] under the same rules as other IETF
 decisions (see [RFC2026]) and MAY result in the authority to maintain
 the extension being withdrawn or reassigned by the IESG.

Phillips & Davis Best Current Practice [Page 50] RFC 5646 Language Tags September 2009

 %%
 Identifier:
 Description:
 Comments:
 Added:
 RFC:
 Authority:
 Contact_Email:
 Mailing_List:
 URL:
 %%
  Figure 6: Format of Records in the Language Tag Extensions Registry
 'Identifier' contains the single-character subtag (singleton)
 assigned to the extension.  The Internet-Draft submitted to define
 the extension SHOULD specify which letter or digit to use, although
 the IESG MAY change the assignment when approving the RFC.
 'Description' contains the name and description of the extension.
 'Comments' is an OPTIONAL field and MAY contain a broader description
 of the extension.
 'Added' contains the date the extension's RFC was published in the
 "full-date" format specified in [RFC3339].  For example: 2004-06-28
 represents June 28, 2004, in the Gregorian calendar.
 'RFC' contains the RFC number assigned to the extension.
 'Authority' contains the name of the maintaining authority for the
 extension.
 'Contact_Email' contains the email address used to contact the
 maintaining authority.
 'Mailing_List' contains the URL or subscription email address of the
 mailing list used by the maintaining authority.
 'URL' contains the URL of the registry for this extension.
 The determination of whether an Internet-Draft meets the above
 conditions and the decision to grant or withhold such authority rests
 solely with the IESG and is subject to the normal review and appeals
 process associated with the RFC process.
 Extension authors are strongly cautioned that many (including most
 well-formed) processors will be unaware of any special relationships

Phillips & Davis Best Current Practice [Page 51] RFC 5646 Language Tags September 2009

 or meaning inherent in the order of extension subtags.  Extension
 authors SHOULD avoid subtag relationships or canonicalization
 mechanisms that interfere with matching or with length restrictions
 that sometimes exist in common protocols where the extension is used.
 In particular, applications MAY truncate the subtags in doing
 matching or in fitting into limited lengths, so it is RECOMMENDED
 that the most significant information be in the most significant
 (left-most) subtags and that the specification gracefully handle
 truncated subtags.
 When a language tag is to be used in a specific, known protocol, it
 is RECOMMENDED that the language tag not contain extensions not
 supported by that protocol.  In addition, note that some protocols
 MAY impose upper limits on the length of the strings used to store or
 transport the language tag.

3.8. Update of the Language Subtag Registry

 After the adoption of this document, the IANA Language Subtag
 Registry needed an update so that it would contain the complete set
 of subtags valid in a language tag.  [RFC5645] describes the process
 used to create this update.
 Registrations that are in process under the rules defined in
 [RFC4646] when this document is adopted MUST be completed under the
 rules contained in this document.

3.9. Applicability of the Subtag Registry

 The Language Subtag Registry is the source of data elements used to
 construct language tags, following the rules described in this
 document.  Language tags are designed for indicating linguistic
 attributes of various content, including not only text but also most
 media formats, such as video or audio.  They also form the basis for
 language and locale negotiation in various protocols and APIs.
 The registry is therefore applicable to many applications that need
 some form of language identification, with these limitations:
 o  It is not designed to be the sole data source in the creation of a
    language-selection user interface.  For example, the registry does
    not contain translations for subtag descriptions or for tags
    composed from the subtags.  Sources for localized data based on
    the registry are generally available, notably [CLDR].  Nor does
    the registry indicate which subtag combinations are particularly
    useful or relevant.

Phillips & Davis Best Current Practice [Page 52] RFC 5646 Language Tags September 2009

 o  It does not provide information indicating relationships between
    different languages, such as might be used in a user interface to
    select language tags hierarchically, regionally, or on some other
    organizational model.
 o  It does not supply information about potential overlap between
    different language tags, as the notion of what constitutes a
    language is not precise: several different language tags might be
    reasonable choices for the same given piece of content.
 o  It does not contain information about appropriate fallback choices
    when performing language negotiation.  A good fallback language
    might be linguistically unrelated to the specified language.  The
    fact that one language is often used as a fallback language for
    another is usually a result of outside factors, such as geography,
    history, or culture -- factors that might not apply in all cases.
    For example, most people who use Breton (a Celtic language used in
    the Northwest of France) would probably prefer to be served French
    (a Romance language) if Breton isn't available.

4. Formation and Processing of Language Tags

 This section addresses how to use the information in the registry
 with the tag syntax to choose, form, and process language tags.

4.1. Choice of Language Tag

 The guiding principle in forming language tags is to "tag content
 wisely."  Sometimes there is a choice between several possible tags
 for the same content.  The choice of which tag to use depends on the
 content and application in question, and some amount of judgment
 might be necessary when selecting a tag.
 Interoperability is best served when the same language tag is used
 consistently to represent the same language.  If an application has
 requirements that make the rules here inapplicable, then that
 application risks damaging interoperability.  It is strongly
 RECOMMENDED that users not define their own rules for language tag
 choice.
 Standards, protocols, and applications that reference this document
 normatively but apply different rules to the ones given in this
 section MUST specify how language tag selection varies from the
 guidelines given here.
 To ensure consistent backward compatibility, this document contains
 several provisions to account for potential instability in the
 standards used to define the subtags that make up language tags.

Phillips & Davis Best Current Practice [Page 53] RFC 5646 Language Tags September 2009

 These provisions mean that no valid language tag can become invalid,
 nor will a language tag have a narrower scope in the future (it may
 have a broader scope).  The most appropriate language tag for a given
 application or content item might evolve over time, but once applied,
 the tag itself cannot become invalid or have its meaning wholly
 change.
 A subtag SHOULD only be used when it adds useful distinguishing
 information to the tag.  Extraneous subtags interfere with the
 meaning, understanding, and processing of language tags.  In
 particular, users and implementations SHOULD follow the 'Prefix' and
 'Suppress-Script' fields in the registry (defined in Section 3.1):
 these fields provide guidance on when specific additional subtags
 SHOULD be used or avoided in a language tag.
 The choice of subtags used to form a language tag SHOULD follow these
 guidelines:
 1.  Use as precise a tag as possible, but no more specific than is
     justified.  Avoid using subtags that are not important for
     distinguishing content in an application.
  • For example, 'de' might suffice for tagging an email written

in German, while "de-CH-1996" is probably unnecessarily

        precise for such a task.
  • Note that some subtag sequences might not represent the

language a casual user might expect. For example, the Swiss

        German (Schweizerdeutsch) language is represented by "gsw-CH"
        and not by "de-CH".  This latter tag represents German ('de')
        as used in Switzerland ('CH'), also known as Swiss High German
        (Schweizer Hochdeutsch).  Both are real languages, and
        distinguishing between them could be important to an
        application.
 2.  The script subtag SHOULD NOT be used to form language tags unless
     the script adds some distinguishing information to the tag.
     Script subtags were first formally defined in [RFC4646].  Their
     use can affect matching and subtag identification for
     implementations of [RFC1766] or [RFC3066] (which are obsoleted by
     this document), as these subtags appear between the primary
     language and region subtags.  Some applications can benefit from
     the use of script subtags in language tags, as long as the use is
     consistent for a given context.  Script subtags are never
     appropriate for unwritten content (such as audio recordings).
     The field 'Suppress-Script' in the primary or extended language
     record in the registry indicates script subtags that do not add
     distinguishing information for most applications; this field

Phillips & Davis Best Current Practice [Page 54] RFC 5646 Language Tags September 2009

     defines when users SHOULD NOT include a script subtag with a
     particular primary language subtag.
     For example, if an implementation selects content using Basic
     Filtering [RFC4647] (originally described in Section 14.4 of
     [RFC2616]) and the user requested the language range "en-US",
     content labeled "en-Latn-US" will not match the request and thus
     not be selected.  Therefore, it is important to know when script
     subtags will customarily be used and when they ought not be used.
     For example:
  • The subtag 'Latn' should not be used with the primary language

'en' because nearly all English documents are written in the

        Latin script and it adds no distinguishing information.
        However, if a document were written in English mixing Latin
        script with another script such as Braille ('Brai'), then it
        might be appropriate to choose to indicate both scripts to aid
        in content selection, such as the application of a style
        sheet.
  • When labeling content that is unwritten (such as a recording

of human speech), the script subtag should not be used, even

        if the language is customarily written in several scripts.
        Thus, the subtitles to a movie might use the tag "uz-Arab"
        (Uzbek, Arabic script), but the audio track for the same
        language would be tagged simply "uz".  (The tag "uz-Zxxx"
        could also be used where content is not written, as the subtag
        'Zxxx' represents the "Code for unwritten documents".)
 3.  If a tag or subtag has a 'Preferred-Value' field in its registry
     entry, then the value of that field SHOULD be used to form the
     language tag in preference to the tag or subtag in which the
     preferred value appears.
  • For example, use 'jbo' for Lojban in preference to the

grandfathered tag "art-lojban".

 4.  Use subtags or sequences of subtags for individual languages in
     preference to subtags for language collections.  A "language
     collection" is a group of languages that are descended from a
     common ancestor, are spoken in the same geographical area, or are
     otherwise related.  Certain language collections are assigned
     codes by [ISO639-5] (and some of these [ISO639-5] codes are also
     defined as collections in [ISO639-2]).  These codes are included
     as primary language subtags in the registry.  Subtags for a
     language collection in the registry have a 'Scope' field with a
     value of 'collection'.  A subtag for a language collection is

Phillips & Davis Best Current Practice [Page 55] RFC 5646 Language Tags September 2009

     always preferred to less specific alternatives such as 'mul' and
     'und' (see below), and a subtag representing a language
     collection MAY be used when more specific language information is
     not available.  However, most users and implementations do not
     know there is a relationship between the collection and its
     individual languages.  In addition, the relationship between the
     individual languages in the collection is not well defined; in
     particular, the languages are usually not mutually intelligible.
     Since the subtags are different, a request for the collection
     will typically only produce items tagged with the collection's
     subtag, not items tagged with subtags for the individual
     languages contained in the collection.
  • For example, collections are interpreted inclusively, so the

subtag 'gem' (Germanic languages) could, but SHOULD NOT, be

        used with content that would be better tagged with "en"
        (English), "de" (German), or "gsw" (Swiss German, Alemannic).
        While 'gem' collects all of these (and other) languages, most
        implementations will not match 'gem' to the individual
        languages; thus, using the subtag will not produce the desired
        result.
 5.  [ISO639-2] has defined several codes included in the subtag
     registry that require additional care when choosing language
     tags.  In most of these cases, where omitting the language tag is
     permitted, such omission is preferable to using these codes.
     Language tags SHOULD NOT incorporate these subtags as a prefix,
     unless the additional information conveys some value to the
     application.
  • The 'mul' (Multiple) primary language subtag identifies

content in multiple languages. This subtag SHOULD NOT be used

        when a list of languages or individual tags for each content
        element can be used instead.  For example, the 'Content-
        Language' header [RFC3282] allows a list of languages to be
        used, not just a single language tag.
  • The 'und' (Undetermined) primary language subtag identifies

linguistic content whose language is not determined. This

        subtag SHOULD NOT be used unless a language tag is required
        and language information is not available or cannot be
        determined.  Omitting the language tag (where permitted) is
        preferred.  The 'und' subtag might be useful for protocols
        that require a language tag to be provided or where a primary
        language subtag is required (such as in "und-Latn").  The
        'und' subtag MAY also be useful when matching language tags in
        certain situations.

Phillips & Davis Best Current Practice [Page 56] RFC 5646 Language Tags September 2009

  • The 'zxx' (Non-Linguistic, Not Applicable) primary language

subtag identifies content for which a language classification

        is inappropriate or does not apply.  Some examples might
        include instrumental or electronic music; sound recordings
        consisting of nonverbal sounds; audiovisual materials with no
        narration, dialog, printed titles, or subtitles; machine-
        readable data files consisting of machine languages or
        character codes; or programming source code.
  • The 'mis' (Uncoded) primary language subtag identifies content

whose language is known but that does not currently have a

        corresponding subtag.  This subtag SHOULD NOT be used.
        Because the addition of other codes in the future can render
        its application invalid, it is inherently unstable and hence
        incompatible with the stability goals of BCP 47.  It is always
        preferable to use other subtags: either 'und' or (with prior
        agreement) private use subtags.
 6.  Use variant subtags sparingly and in the correct order.  Most
     variant subtags have one or more 'Prefix' fields in the registry
     that express the list of subtags with which they are appropriate.
     Variants SHOULD only be used with subtags that appear in one of
     these 'Prefix' fields.  If a variant lists a second variant in
     one of its 'Prefix' fields, the first variant SHOULD appear
     directly after the second variant in any language tag where both
     occur.  General purpose variants (those with no 'Prefix' fields
     at all) SHOULD appear after any other variant subtags.  Order any
     remaining variants by placing the most significant subtag first.
     If none of the subtags is more significant or no relationship can
     be determined, alphabetize the subtags.  Because variants are
     very specialized, using many of them together generally makes the
     tag so narrow as to override the additional precision gained.
     Putting the subtags into another order interferes with
     interoperability, as well as the overall interpretation of the
     tag.
     For example:
  • The tag "en-scotland-fonipa" (English, Scottish dialect, IPA

phonetic transcription) is correctly ordered because

        'scotland' has a 'Prefix' of "en", while 'fonipa' has no
        'Prefix' field.
  • The tag "sl-IT-rozaj-biske-1994" is correctly ordered: 'rozaj'

lists "sl" as its sole 'Prefix'; 'biske' lists "sl-rozaj" as

        its sole 'Prefix'.  The subtag '1994' has several prefixes,

Phillips & Davis Best Current Practice [Page 57] RFC 5646 Language Tags September 2009

        including "sl-rozaj".  However, it follows both 'rozaj' and
        'biske' because one of its 'Prefix' fields is "sl-rozaj-
        biske".
 7.  The grandfathered tag "i-default" (Default Language) was
     originally registered according to [RFC1766] to meet the needs of
     [RFC2277].  It is not used to indicate a specific language, but
     rather to identify the condition or content used where the
     language preferences of the user cannot be established.  It
     SHOULD NOT be used except as a means of labeling the default
     content for applications or protocols that require default
     language content to be labeled with that specific tag.  It MAY
     also be used by an application or protocol to identify when the
     default language content is being returned.

4.1.1. Tagging Encompassed Languages

 Some primary language records in the registry have a 'Macrolanguage'
 field (Section 3.1.10) that contains a mapping from each "encompassed
 language" to its macrolanguage.  The 'Macrolanguage' mapping doesn't
 define what the relationship between the encompassed language and its
 macrolanguage is, nor does it define how languages encompassed by the
 same macrolanguage are related to each other.  Two different
 languages encompassed by the same macrolanguage may differ from one
 another more than, say, French and Spanish do.
 A few specific macrolanguages, such as Chinese ('zh') and Arabic
 ('ar'), are handled differently.  See Section 4.1.2.
 The more specific encompassed language subtag SHOULD be used to form
 the language tag, although either the macrolanguage's primary
 language subtag or the encompassed language's subtag MAY be used.
 This means, for example, tagging Plains Cree with 'crk' rather than
 'cr' (Cree), and so forth.
 Each macrolanguage subtag's scope, by definition, includes all of its
 encompassed languages.  Since the relationship between encompassed
 languages varies, users cannot assume that the macrolanguage subtag
 means any particular encompassed language, nor that any given pair of
 encompassed languages are mutually intelligible or otherwise
 interchangeable.
 Applications MAY use macrolanguage information to improve matching or
 language negotiation.  For example, the information that 'sr'
 (Serbian) and 'hr' (Croatian) share a macrolanguage expresses a
 closer relation between those languages than between, say, 'sr'
 (Serbian) and 'ma' (Macedonian).  However, this relationship is not
 guaranteed nor is it exclusive.  For example, Romanian ('ro') and

Phillips & Davis Best Current Practice [Page 58] RFC 5646 Language Tags September 2009

 Moldavian ('mo') do not share a macrolanguage, but are far more
 closely related to each other than Cantonese ('yue') and Wu ('wuu'),
 which do share a macrolanguage.

4.1.2. Using Extended Language Subtags

 To accommodate language tag forms used prior to the adoption of this
 document, language tags provide a special compatibility mechanism:
 the extended language subtag.  Selected languages have been provided
 with both primary and extended language subtags.  These include
 macrolanguages, such as Malay ('ms') and Uzbek ('uz'), that have a
 specific dominant variety that is generally synonymous with the
 macrolanguage.  Other languages, such as the Chinese ('zh') and
 Arabic ('ar') macrolanguages and the various sign languages ('sgn'),
 have traditionally used their primary language subtag, possibly
 coupled with various region subtags or as part of a registered
 grandfathered tag, to indicate the language.
 With the adoption of this document, specific ISO 639-3 subtags became
 available to identify the languages contained within these diverse
 language families or groupings.  This presents a choice of language
 tags where previously none existed:
 o  Each encompassed language's subtag SHOULD be used as the primary
    language subtag.  For example, a document in Mandarin Chinese
    would be tagged "cmn" (the subtag for Mandarin Chinese) in
    preference to "zh" (Chinese).
 o  If compatibility is desired or needed, the encompassed subtag MAY
    be used as an extended language subtag.  For example, a document
    in Mandarin Chinese could be tagged "zh-cmn" instead of either
    "cmn" or "zh".
 o  The macrolanguage or prefixing subtag MAY still be used to form
    the tag instead of the more specific encompassed language subtag.
    That is, tags such as "zh-HK" or "sgn-RU" are still valid.
 Chinese ('zh') provides a useful illustration of this.  In the past,
 various content has used tags beginning with the 'zh' subtag, with
 application-specific meaning being associated with region codes,
 private use sequences, or grandfathered registered values.  This is
 because historically only the macrolanguage subtag 'zh' was available
 for forming language tags.  However, the languages encompassed by the
 Chinese subtag 'zh' are, in the main, not mutually intelligible when
 spoken, and the written forms of these languages also show wide
 variation in form and usage.

Phillips & Davis Best Current Practice [Page 59] RFC 5646 Language Tags September 2009

 To provide compatibility, Chinese languages encompassed by the 'zh'
 subtag are in the registry both as primary language subtags and as
 extended language subtags.  For example, the ISO 639-3 code for
 Cantonese is 'yue'.  Content in Cantonese might historically have
 used a tag such as "zh-HK" (since Cantonese is commonly spoken in
 Hong Kong), although that tag actually means any type of Chinese as
 used in Hong Kong.  With the availability of ISO 639-3 codes in the
 registry, content in Cantonese can be directly tagged using the 'yue'
 subtag.  The content can use it as a primary language subtag, as in
 the tag "yue-HK" (Cantonese, Hong Kong).  Or it can use an extended
 language subtag with 'zh', as in the tag "zh-yue-Hant" (Chinese,
 Cantonese, Traditional script).
 As noted above, applications can choose to use the macrolanguage
 subtag to form the tag instead of using the more specific encompassed
 language subtag.  For example, an application with large quantities
 of data already using tags with the 'zh' (Chinese) subtag might
 continue to use this more general subtag even for new data, even
 though the content could be more precisely tagged with 'cmn'
 (Mandarin), 'yue' (Cantonese), 'wuu' (Wu), and so on.  Similarly, an
 application already using tags that start with the 'ar' (Arabic)
 subtag might continue to use this more general subtag even for new
 data, which could be more precisely tagged with 'arb' (Standard
 Arabic).
 In some cases, the encompassed languages had tags registered for them
 during the RFC 3066 era.  Those grandfathered tags not already
 deprecated or rendered redundant were deprecated in the registry upon
 adoption of this document.  As grandfathered values, they remain
 valid for use, and some content or applications might use them.  As
 with other grandfathered tags, since implementations might not be
 able to associate the grandfathered tags with the encompassed
 language subtag equivalents that are recommended by this document,
 implementations are encouraged to canonicalize tags for comparison
 purposes.  Some examples of this include the tags "zh-hakka" (Hakka)
 and "zh-guoyu" (Mandarin or Standard Chinese).
 Sign languages share a mode of communication rather than a linguistic
 heritage.  There are many sign languages that have developed
 independently, and the subtag 'sgn' indicates only the presence of a
 sign language.  A number of sign languages also had grandfathered
 tags registered for them during the RFC 3066 era.  For example, the
 grandfathered tag "sgn-US" was registered to represent 'American Sign
 Language' specifically, without reference to the United States.  This
 is still valid, but deprecated: a document in American Sign Language
 can be labeled either "ase" or "sgn-ase" (the 'ase' subtag is for the
 language called 'American Sign Language').

Phillips & Davis Best Current Practice [Page 60] RFC 5646 Language Tags September 2009

4.2. Meaning of the Language Tag

 The meaning of a language tag is related to the meaning of the
 subtags that it contains.  Each subtag, in turn, implies a certain
 range of expectations one might have for related content, although it
 is not a guarantee.  For example, the use of a script subtag such as
 'Arab' (Arabic script) does not mean that the content contains only
 Arabic characters.  It does mean that the language involved is
 predominantly in the Arabic script.  Thus, a language tag and its
 subtags can encompass a very wide range of variation and yet remain
 appropriate in each particular instance.
 Validity of a tag is not the only factor determining its usefulness.
 While every valid tag has a meaning, it might not represent any real-
 world language usage.  This is unavoidable in a system in which
 subtags can be combined freely.  For example, tags such as
 "ar-Cyrl-CO" (Arabic, Cyrillic script, as used in Colombia) or "tlh-
 Kore-AQ-fonipa" (Klingon, Korean script, as used in Antarctica, IPA
 phonetic transcription) are both valid and unlikely to represent a
 useful combination of language attributes.
 The meaning of a given tag doesn't depend on the context in which it
 appears.  The relationship between a tag's meaning and the
 information objects to which that tag is applied, however, can vary.
 o  For a single information object, the associated language tags
    might be interpreted as the set of languages that is necessary for
    a complete comprehension of the complete object.  Example: Plain
    text documents.
 o  For an aggregation of information objects, the associated language
    tags could be taken as the set of languages used inside components
    of that aggregation.  Examples: Document stores and libraries.
 o  For information objects whose purpose is to provide alternatives,
    the associated language tags could be regarded as a hint that the
    content is provided in several languages and that one has to
    inspect each of the alternatives in order to find its language or
    languages.  In this case, the presence of multiple tags might not
    mean that one needs to be multilingual to get complete
    understanding of the document.  Example: MIME multipart/
    alternative [RFC2046].
 o  For markup languages, such as HTML and XML, language information
    can be added to each part of the document identified by the markup
    structure (including the whole document itself).  For example, one
    could write <span lang="fr">C'est la vie.</span> inside a German
    document; the German-speaking user could then access a French-

Phillips & Davis Best Current Practice [Page 61] RFC 5646 Language Tags September 2009

    German dictionary to find out what the marked section meant.  If
    the user were listening to that document through a speech
    synthesis interface, this formation could be used to signal the
    synthesizer to appropriately apply French text-to-speech
    pronunciation rules to that span of text, instead of applying the
    inappropriate German rules.
 o  For markup languages and document formats that allow the audience
    to be identified, a language tag could indicate the audience(s)
    appropriate for that document.  For example, the same HTML
    document described in the preceding bullet might have an HTTP
    header "Content-Language: de" to indicate that the intended
    audience for the file is German (even though three words appear
    and are identified as being in French within it).
 o  For systems and APIs, language tags form the basis for most
    implementations of locale identifiers.  For example, see Unicode's
    CLDR (Common Locale Data Repository) (see UTS #35 [UTS35])
    project.
 Language tags are related when they contain a similar sequence of
 subtags.  For example, if a language tag B contains language tag A as
 a prefix, then B is typically "narrower" or "more specific" than A.
 Thus, "zh-Hant-TW" is more specific than "zh-Hant".
 This relationship is not guaranteed in all cases: specifically,
 languages that begin with the same sequence of subtags are NOT
 guaranteed to be mutually intelligible, although they might be.  For
 example, the tag "az" shares a prefix with both "az-Latn"
 (Azerbaijani written using the Latin script) and "az-Cyrl"
 (Azerbaijani written using the Cyrillic script).  A person fluent in
 one script might not be able to read the other, even though the
 linguistic content (e.g., what would be heard if both texts were read
 aloud) might be identical.  Content tagged as "az" most probably is
 written in just one script and thus might not be intelligible to a
 reader familiar with the other script.
 Similarly, not all subtags specify an actual distinction in language.
 For example, the tags "en-US" and "en-CA" mean, roughly, English with
 features generally thought to be characteristic of the United States
 and Canada, respectively.  They do not imply that a significant
 dialectical boundary exists between any arbitrarily selected point in
 the United States and any arbitrarily selected point in Canada.
 Neither does a particular region subtag imply that linguistic
 distinctions do not exist within that region.

Phillips & Davis Best Current Practice [Page 62] RFC 5646 Language Tags September 2009

4.3. Lists of Languages

 In some applications, a single content item might best be associated
 with more than one language tag.  Examples of such a usage include:
 o  Content items that contain multiple, distinct varieties.  Often
    this is used to indicate an appropriate audience for a given
    content item when multiple choices might be appropriate.  Examples
    of this could include:
  • Metadata about the appropriate audience for a movie title. For

example, a DVD might label its individual audio tracks 'de'

       (German), 'fr' (French), and 'es' (Spanish), but the overall
       title would list "de, fr, es" as its overall audience.
  • A French/English, English/French dictionary tagged as both "en"

and "fr" to specify that it applies equally to French and

       English.
  • A side-by-side or interlinear translation of a document, as is

commonly done with classical works in Latin or Greek.

 o  Content items that contain a single language but that require
    multiple levels of specificity.  For example, a library might wish
    to classify a particular work as both Norwegian ('no') and as
    Nynorsk ('nn') for audiences capable of appreciating the
    distinction or needing to select content more narrowly.

4.4. Length Considerations

 There is no defined upper limit on the size of language tags.  While
 historically most language tags have consisted of language and region
 subtags with a combined total length of up to six characters, larger
 tags have always been both possible and have actually appeared in
 use.
 Neither the language tag syntax nor other requirements in this
 document impose a fixed upper limit on the number of subtags in a
 language tag (and thus an upper bound on the size of a tag).  The
 language tag syntax suggests that, depending on the specific
 language, more subtags (and thus a longer tag) are sometimes
 necessary to completely identify the language for certain
 applications; thus, it is possible to envision long or complex subtag
 sequences.

Phillips & Davis Best Current Practice [Page 63] RFC 5646 Language Tags September 2009

4.4.1. Working with Limited Buffer Sizes

 Some applications and protocols are forced to allocate fixed buffer
 sizes or otherwise limit the length of a language tag.  A conformant
 implementation or specification MAY refuse to support the storage of
 language tags that exceed a specified length.  Any such limitation
 SHOULD be clearly documented, and such documentation SHOULD include
 what happens to longer tags (for example, whether an error value is
 generated or the language tag is truncated).  A protocol that allows
 tags to be truncated at an arbitrary limit, without giving any
 indication of what that limit is, has the potential to cause harm by
 changing the meaning of tags in substantial ways.
 In practice, most language tags do not require more than a few
 subtags and will not approach reasonably sized buffer limitations;
 see Section 4.1.
 Some specifications or protocols have limits on tag length but do not
 have a fixed length limitation.  For example, [RFC2231] has no
 explicit length limitation: the length available for the language tag
 is constrained by the length of other header components (such as the
 charset's name) coupled with the 76-character limit in [RFC2047].
 Thus, the "limit" might be 50 or more characters, but it could
 potentially be quite small.
 The considerations for assigning a buffer limit are:
    Implementations SHOULD NOT truncate language tags unless the
    meaning of the tag is purposefully being changed, or unless the
    tag does not fit into a limited buffer size specified by a
    protocol for storage or transmission.
    Implementations SHOULD warn the user when a tag is truncated since
    truncation changes the semantic meaning of the tag.
    Implementations of protocols or specifications that are space
    constrained but do not have a fixed limit SHOULD use the longest
    possible tag in preference to truncation.
    Protocols or specifications that specify limited buffer sizes for
    language tags MUST allow for language tags of at least 35
    characters.  Note that [RFC4646] recommended a minimum field size
    of 42 characters because it included all three elements of the
    'extlang' production.  Two of these are now permanently reserved,
    so a registered primary language subtag of the maximum length of 8
    characters is now longer than the longest language-extlang
    combination.  Protocols or specifications that commonly use

Phillips & Davis Best Current Practice [Page 64] RFC 5646 Language Tags September 2009

    extensions or private use subtags might wish to reserve or
    recommend a longer "minimum buffer" size.
 The following illustration shows how the 35-character recommendation
 was derived:
 language      =  8 ; longest allowed registered value
                    ;   longer than primary+extlang
                    ;   which requires 7 characters
 script        =  5 ; if not suppressed: see Section 4.1
 region        =  4 ; UN M.49 numeric region code
                    ;   ISO 3166-1 codes require 3
 variant1      =  9 ; needs 'language' as a prefix
 variant2      =  9 ; very rare, as it needs
                    ;   'language-variant1' as a prefix
 total         = 35 characters
            Figure 7: Derivation of the Limit on Tag Length

4.4.2. Truncation of Language Tags

 Truncation of a language tag alters the meaning of the tag, and thus
 SHOULD be avoided.  However, truncation of language tags is sometimes
 necessary due to limited buffer sizes.  Such truncation MUST NOT
 permit a subtag to be chopped off in the middle or the formation of
 invalid tags (for example, one ending with the "-" character).
 This means that applications or protocols that truncate tags MUST do
 so by progressively removing subtags along with their preceding "-"
 from the right side of the language tag until the tag is short enough
 for the given buffer.  If the resulting tag ends with a single-
 character subtag, that subtag and its preceding "-" MUST also be
 removed.  For example:
 Tag to truncate: zh-Latn-CN-variant1-a-extend1-x-wadegile-private1
 1. zh-Latn-CN-variant1-a-extend1-x-wadegile
 2. zh-Latn-CN-variant1-a-extend1
 3. zh-Latn-CN-variant1
 4. zh-Latn-CN
 5. zh-Latn
 6. zh
                  Figure 8: Example of Tag Truncation

Phillips & Davis Best Current Practice [Page 65] RFC 5646 Language Tags September 2009

4.5. Canonicalization of Language Tags

 Since a particular language tag can be used by many processes,
 language tags SHOULD always be created or generated in canonical
 form.
 A language tag is in 'canonical form' when the tag is well-formed
 according to the rules in Sections 2.1 and 2.2 and it has been
 canonicalized by applying each of the following steps in order, using
 data from the IANA registry (see Section 3.1):
 1.  Extension sequences are ordered into case-insensitive ASCII order
     by singleton subtag.
  • For example, the subtag sequence '-a-babble' comes before

'-b-warble'.

 2.  Redundant or grandfathered tags are replaced by their 'Preferred-
     Value', if there is one.
  • The field-body of the 'Preferred-Value' for grandfathered and

redundant tags is an "extended language range" [RFC4647] and

        might consist of more than one subtag.
  • 'Preferred-Value' fields in the registry provide mappings from

deprecated tags to modern equivalents. Many of these were

        created before the adoption of this document (such as the
        mapping of "no-nyn" to "nn" or "i-klingon" to "tlh").  Others
        are the result of later registrations or additions to the
        registry as permitted or required by this document (for
        example, "zh-hakka" was deprecated in favor of the ISO 639-3
        code 'hak' when this document was adopted).
 3.  Subtags are replaced by their 'Preferred-Value', if there is one.
     For extlangs, the original primary language subtag is also
     replaced if there is a primary language subtag in the 'Preferred-
     Value'.
  • The field-body of the 'Preferred-Value' for extlangs is an

"extended language range" and typically maps to a primary

        language subtag.  For example, the subtag sequence "zh-hak"
        (Chinese, Hakka) is replaced with the subtag 'hak' (Hakka).
  • Most of the non-extlang subtags are either Region subtags

where the country name or designation has changed or clerical

        corrections to ISO 639-1.

Phillips & Davis Best Current Practice [Page 66] RFC 5646 Language Tags September 2009

 The canonical form contains no 'extlang' subtags.  There is an
 alternate 'extlang form' that maintains or reinstates extlang
 subtags.  This form can be useful in environments where the presence
 of the 'Prefix' subtag is considered beneficial in matching or
 selection (see Section 4.1.2).
 A language tag is in 'extlang form' when the tag is well-formed
 according to the rules in Sections 2.1 and 2.2 and it has been
 processed by applying each of the following two steps in order, using
 data from the IANA registry:
 1.  The language tag is first transformed into canonical form, as
     described above.
 2.  If the language tag starts with a primary language subtag that is
     also an extlang subtag, then the language tag is prepended with
     the extlang's 'Prefix'.
  • For example, "hak-CN" (Hakka, China) has the primary language

subtag 'hak', which in turn has an 'extlang' record with a

        'Prefix' 'zh' (Chinese).  The extlang form is "zh-hak-CN"
        (Chinese, Hakka, China).
  • Note that Step 2 (prepending a prefix) can restore a subtag

that was removed by Step 1 (canonicalizing).

 Example: The language tag "en-a-aaa-b-ccc-bbb-x-xyz" is in canonical
 form, while "en-b-ccc-bbb-a-aaa-X-xyz" is well-formed and potentially
 valid (extensions 'a' and 'b' are not defined as of the publication
 of this document) but not in canonical form (the extensions are not
 in alphabetical order).
 Example: Although the tag "en-BU" (English as used in Burma)
 maintains its validity, the language tag "en-BU" is not in canonical
 form because the 'BU' subtag has a canonical mapping to 'MM'
 (Myanmar).
 Canonicalization of language tags does not imply anything about the
 use of upper- or lowercase letters when processing or comparing
 subtags (and as described in Section 2.1).  All comparisons MUST be
 performed in a case-insensitive manner.
 When performing canonicalization of language tags, processors MAY
 regularize the case of the subtags (that is, this process is
 OPTIONAL), following the case used in the registry (see
 Section 2.1.1).

Phillips & Davis Best Current Practice [Page 67] RFC 5646 Language Tags September 2009

 If more than one variant appears within a tag, processors MAY reorder
 the variants to obtain better matching behavior or more consistent
 presentation.  Reordering of the variants SHOULD follow the
 recommendations for variant ordering in Section 4.1.
 If the field 'Deprecated' appears in a registry record without an
 accompanying 'Preferred-Value' field, then that tag or subtag is
 deprecated without a replacement.  These values are canonical when
 they appear in a language tag.  However, tags that include these
 values SHOULD NOT be selected by users or generated by
 implementations.
 An extension MUST define any relationships that exist between the
 various subtags in the extension and thus MAY define an alternate
 canonicalization scheme for the extension's subtags.  Extensions MAY
 define how the order of the extension's subtags is interpreted.  For
 example, an extension could define that its subtags are in canonical
 order when the subtags are placed into ASCII order: that is, "en-a-
 aaa-bbb-ccc" instead of "en-a-ccc-bbb-aaa".  Another extension might
 define that the order of the subtags influences their semantic
 meaning (so that "en-b-ccc-bbb-aaa" has a different value from "en-b-
 aaa-bbb-ccc").  However, extension specifications SHOULD be designed
 so that they are tolerant of the typical processes described in
 Section 3.7.

4.6. Considerations for Private Use Subtags

 Private use subtags, like all other subtags, MUST conform to the
 format and content constraints in the ABNF.  Private use subtags have
 no meaning outside the private agreement between the parties that
 intend to use or exchange language tags that employ them.  The same
 subtags MAY be used with a different meaning under a separate private
 agreement.  They SHOULD NOT be used where alternatives exist and
 SHOULD NOT be used in content or protocols intended for general use.
 Private use subtags are simply useless for information exchange
 without prior arrangement.  The value and semantic meaning of private
 use tags and of the subtags used within such a language tag are not
 defined by this document.
 Private use sequences introduced by the 'x' singleton are completely
 opaque to users or implementations outside of the private use
 agreement.  So, in addition to private use subtag sequences
 introduced by the singleton subtag 'x', the Language Subtag Registry
 provides private use language, script, and region subtags derived
 from the private use codes assigned by the underlying standards.
 These subtags are valid for use in forming language tags; they are
 RECOMMENDED over the 'x' singleton private use subtag sequences

Phillips & Davis Best Current Practice [Page 68] RFC 5646 Language Tags September 2009

 because they convey more information via their linkage to the
 language tag's inherent structure.
 For example, the region subtags 'AA', 'ZZ', and those in the ranges
 'QM'-'QZ' and 'XA'-'XZ' (derived from the ISO 3166-1 private use
 codes) can be used to form a language tag.  A tag such as
 "zh-Hans-XQ" conveys a great deal of public, interchangeable
 information about the language material (that it is Chinese in the
 simplified Chinese script and is suitable for some geographic region
 'XQ').  While the precise geographic region is not known outside of
 private agreement, the tag conveys far more information than an
 opaque tag such as "x-somelang" or even "zh-Hans-x-xq" (where the
 'xq' subtag's meaning is entirely opaque).
 However, in some cases content tagged with private use subtags can
 interact with other systems in a different and possibly unsuitable
 manner compared to tags that use opaque, privately defined subtags,
 so the choice of the best approach sometimes depends on the
 particular domain in question.

5. IANA Considerations

 This section deals with the processes and requirements necessary for
 IANA to maintain the subtag and extension registries as defined by
 this document and in accordance with the requirements of [RFC5226].
 The impact on the IANA maintainers of the two registries defined by
 this document will be a small increase in the frequency of new
 entries or updates.  IANA also is required to create a new mailing
 list (described below in Section 5.1) to announce registry changes
 and updates.

5.1. Language Subtag Registry

 IANA updated the registry using instructions and content provided in
 a companion document [RFC5645].  The criteria and process for
 selecting the updated set of records are described in that document.
 The updated set of records represents no impact on IANA, since the
 work to create it will be performed externally.
 Future work on the Language Subtag Registry includes the following
 activities:
 o  Inserting or replacing whole records.  These records are
    preformatted for IANA by the Language Subtag Reviewer, as
    described in Section 3.3.
 o  Archiving and making publicly available the registration forms.

Phillips & Davis Best Current Practice [Page 69] RFC 5646 Language Tags September 2009

 o  Announcing each updated version of the registry on the
    "ietf-languages-announcements@iana.org" mailing list.
 Each registration form sent to IANA contains a single record for
 incorporation into the registry.  The form will be sent to
 <iana@iana.org> by the Language Subtag Reviewer.  It will have a
 subject line indicating whether the enclosed form represents an
 insertion of a new record (indicated by the word "INSERT" in the
 subject line) or a replacement of an existing record (indicated by
 the word "MODIFY" in the subject line).  At no time can a record be
 deleted from the registry.
 IANA will extract the record from the form and place the inserted or
 modified record into the appropriate section of the Language Subtag
 Registry, grouping the records by their 'Type' field.  Inserted
 records can be placed anywhere within the appropriate section; there
 is no guarantee that the registry's records will be placed in any
 particular order except that they will always be grouped by 'Type'.
 Modified records overwrite the record they replace.
 Whenever an entry is created or modified in the registry, the 'File-
 Date' record at the start of the registry is updated to reflect the
 most recent modification date.  The date format SHALL be the "full-
 date" format of [RFC3339].  The date SHALL be the date on which that
 version of the registry was first published by IANA.  There SHALL be
 at most one version of the registry published in a day.  A 'File-
 Date' record is also included in each request to IANA to insert or
 modify records, indicating the acceptance date of the records in the
 request.
 The updated registry file MUST use the UTF-8 character encoding, and
 IANA MUST check the registry file for proper encoding.  Non-ASCII
 characters can be sent to IANA by attaching the registration form to
 the email message or by using various encodings in the mail message
 body (UTF-8 is recommended).  IANA will verify any unclear or
 corrupted characters with the Language Subtag Reviewer prior to
 posting the updated registry.
 IANA will also archive and make publicly available from
 http://www.iana.org each registration form.  Note that multiple
 registrations can pertain to the same record in the registry.
 Developers who are dependent upon the Language Subtag Registry
 sometimes would like to be informed of changes in the registry so
 that they can update their implementations.  When any change is made
 to the Language Subtag Registry, IANA will send an announcement
 message to <ietf-languages-announcements@iana.org> (a self-
 subscribing list to which only IANA can post).

Phillips & Davis Best Current Practice [Page 70] RFC 5646 Language Tags September 2009

5.2. Extensions Registry

 The Language Tag Extensions Registry can contain at most 35 records,
 and thus changes to this registry are expected to be very infrequent.
 Future work by IANA on the Language Tag Extensions Registry is
 limited to two cases.  First, the IESG MAY request that new records
 be inserted into this registry from time to time.  These requests
 MUST include the record to insert in the exact format described in
 Section 3.7.  In addition, there MAY be occasional requests from the
 maintaining authority for a specific extension to update the contact
 information or URLs in the record.  These requests MUST include the
 complete, updated record.  IANA is not responsible for validating the
 information provided, only that it is properly formatted.  IANA
 SHOULD take reasonable steps to ascertain that the request comes from
 the maintaining authority named in the record present in the
 registry.

6. Security Considerations

 Language tags used in content negotiation, like any other information
 exchanged on the Internet, might be a source of concern because they
 might be used to infer the nationality of the sender, and thus
 identify potential targets for surveillance.
 This is a special case of the general problem that anything sent is
 visible to the receiving party and possibly to third parties as well.
 It is useful to be aware that such concerns can exist in some cases.
 The evaluation of the exact magnitude of the threat, and any possible
 countermeasures, is left to each application protocol (see BCP 72
 [RFC3552] for best current practice guidance on security threats and
 defenses).
 The language tag associated with a particular information item is of
 no consequence whatsoever in determining whether that content might
 contain possible homographs.  The fact that a text is tagged as being
 in one language or using a particular script subtag provides no
 assurance whatsoever that it does not contain characters from scripts
 other than the one(s) associated with or specified by that language
 tag.
 Since there is no limit to the number of variant, private use, and
 extension subtags, and consequently no limit on the possible length
 of a tag, implementations need to guard against buffer overflow
 attacks.  See Section 4.4 for details on language tag truncation,
 which can occur as a consequence of defenses against buffer overflow.

Phillips & Davis Best Current Practice [Page 71] RFC 5646 Language Tags September 2009

 To prevent denial-of-service attacks, applications SHOULD NOT depend
 on either the Language Subtag Registry or the Language Tag Extensions
 Registry being always accessible.  Additionally, although the
 specification of valid subtags for an extension (see Section 3.7)
 MUST be available over the Internet, implementations SHOULD NOT
 mechanically depend on those sources being always accessible.
 The registries specified in this document are not suitable for
 frequent or real-time access to, or retrieval of, the full registry
 contents.  Most applications do not need registry data at all.  For
 others, being able to validate or canonicalize language tags as of a
 particular registry date will be sufficient, as the registry contents
 change only occasionally.  Changes are announced to
 <ietf-languages-announcements@iana.org>.  This mailing list is
 intended for interested organizations and individuals, not for bulk
 subscription to trigger automatic software updates.  The size of the
 registry makes it unsuitable for automatic software updates.
 Implementers considering integrating the Language Subtag Registry in
 an automatic updating scheme are strongly advised to distribute only
 suitably encoded differences, and only via their own infrastructure
 -- not directly from IANA.
 Changes, or the absence thereof, can also easily be detected by
 looking at the 'File-Date' record at the start of the registry, or by
 using features of the protocol used for downloading, without having
 to download the full registry.  At the time of publication of this
 document, IANA is making the Language Tag Registry available over
 HTTP 1.1.  The proper way to update a local copy of the Language
 Subtag Registry using HTTP 1.1 is to use a conditional GET [RFC2616].

7. Character Set Considerations

 The syntax in this document requires that language tags use only the
 characters A-Z, a-z, 0-9, and HYPHEN-MINUS, which are present in most
 character sets, so the composition of language tags shouldn't have
 any character set issues.
 The rendering of text based on the language tag is not addressed
 here.  Historically, some processes have relied on the use of
 character set/encoding information (or other external information) in
 order to infer how a specific string of characters should be
 rendered.  Notably, this applies to language- and culture-specific
 variations of Han ideographs as used in Japanese, Chinese, and
 Korean, where use of, for example, a Japanese character encoding such
 as EUC-JP implies that the text itself is in Japanese.  When language
 tags are applied to spans of text, rendering engines might be able to
 use that information to better select fonts or make other rendering

Phillips & Davis Best Current Practice [Page 72] RFC 5646 Language Tags September 2009

 choices, particularly where languages with distinct writing
 traditions use the same characters.

8. Changes from RFC 4646

 The main goal for this revision of RFC 4646 was to incorporate two
 new parts of ISO 639 (ISO 639-3 and ISO 639-5) and their attendant
 sets of language codes into the IANA Language Subtag Registry.  This
 permits the identification of many more languages and language
 collections than previously supported.
 The specific changes in this document to meet these goals are:
 o  Defined the incorporation of ISO 639-3 and ISO 639-5 codes for use
    as primary and extended language subtags.  It also permanently
    reserves and disallows the use of additional 'extlang' subtags.
    The changes necessary to achieve this were:
  • Modified the ABNF comments.
  • Updated various registration and stability requirements

sections to reference ISO 639-3 and ISO 639-5 in addition to

       ISO 639-1 and ISO 639-2.
  • Edited the text to eliminate references to extended language

subtags where they are no longer used.

  • Explained the change in the section on extended language

subtags.

 o  Changed the ABNF related to grandfathered tags.  The irregular
    tags are now listed.  Well-formed grandfathered tags are now
    described by the 'langtag' production, and the 'grandfathered'
    production was removed as a result.  Also: added description of
    both types of grandfathered tags to Section 2.2.8.
 o  Added the paragraph on "collections" to Section 4.1.
 o  Changed the capitalization rules for 'Tag' fields in Section 3.1.
 o  Split Section 3.1 up into subsections.
 o  Modified Section 3.5 to allow 'Suppress-Script' fields to be
    added, modified, or removed via the registration process.  This
    was an erratum from RFC 4646.
 o  Modified examples that used region code 'CS' (formerly Serbia and
    Montenegro) to use 'RS' (Serbia) instead.

Phillips & Davis Best Current Practice [Page 73] RFC 5646 Language Tags September 2009

 o  Modified the rules for creating and maintaining record
    'Description' fields to prevent duplicates, including inverted
    duplicates.
 o  Removed the lengthy description of why RFC 4646 was created from
    this section, which also caused the removal of the reference to
    XML Schema.
 o  Modified the text in Section 2.1 to place more emphasis on the
    fact that language tags are not case sensitive.
 o  Replaced the example "fr-Latn-CA" in Section 2.1 with "sr-Latn-RS"
    and "az-Arab-IR" because "fr-Latn-CA" doesn't respect the
    'Suppress-Script' on 'Latn' with 'fr'.
 o  Changed the requirements for well-formedness to make singleton
    repetition checking optional (it is required for validity
    checking) in Section 2.2.9.
 o  Changed the text in Section 2.2.9 referring to grandfathered
    checking to note that the list is now included in the ABNF.
 o  Modified and added text to Section 3.2.  The job description was
    placed first.  A note was added making clear that the Language
    Subtag Reviewer may delegate various non-critical duties,
    including list moderation.  Finally, additional text was added to
    make the appointment process clear and to clarify that decisions
    and performance of the reviewer are appealable.
 o  Added text to Section 3.5 clarifying that the
    ietf-languages@iana.org list is operated by whomever the IESG
    appoints.
 o  Added text to Section 3.1.5 clarifying that the first Description
    in a 'language' record matches the corresponding Reference Name
    for the language in ISO 639-3.
 o  Modified Section 2.2.9 to define classes of conformance related to
    specific tags (formerly 'well-formed' and 'valid' referred to
    implementations).  Notes were added about the removal of 'extlang'
    from the ABNF provided in RFC 4646, allowing for well-formedness
    using this older definition.  Reference to RFC 3066 well-
    formedness was also added.
 o  Added text to the end of Section 3.1.2 noting that future versions
    of this document might add new field types to the registry format
    and recommending that implementations ignore any unrecognized
    fields.

Phillips & Davis Best Current Practice [Page 74] RFC 5646 Language Tags September 2009

 o  Added text about what the lack of a 'Suppress-Script' field means
    in a record to Section 3.1.9.
 o  Added text allowing the correction of misspellings and typographic
    errors to Section 3.1.5.
 o  Added text to Section 3.1.8 disallowing 'Prefix' field conflicts
    (such as circular prefix references).
 o  Modified text in Section 3.5 to require the subtag reviewer to
    announce his/her decision (or extension) following the two-week
    period.  Also clarified that any decision or failure to decide can
    be appealed.
 o  Modified text in Section 4.1 to include the (heretofore anecdotal)
    guiding principle of tag choice, and clarifying the non-use of
    script subtags in non-written applications.
 o  Prohibited multiple use of the same variant in a tag (i.e., "de-
    1901-1901").  Previously, this was only a recommendation
    ("SHOULD").
 o  Removed inappropriate [RFC2119] language from the illustration in
    Section 4.4.1.
 o  Replaced the example of deprecating "zh-guoyu" with "zh-
    hakka"->"hak" in Section 4.5, noting that it was this document
    that caused the change.
 o  Replaced the section in Section 4.1 dealing with "mul"/"und" to
    include the subtags 'zxx' and 'mis', as well as the tag
    "i-default".  A normative reference to RFC 2277 was added.
 o  Added text to Section 3.5 clarifying that any modifications of a
    registration request must be sent to the <ietf-languages@iana.org>
    list before submission to IANA.
 o  Changed the ABNF for the record-jar format from using the LWSP
    production to use a folding whitespace production similar to obs-
    FWS in [RFC5234].  This effectively prevents unintentional blank
    lines inside a field.
 o  Clarified and revised text in Sections 3.3, 3.5, and 5.1 to
    clarify that the Language Subtag Reviewer sends the complete
    registration forms to IANA, that IANA extracts the record from the
    form, and that the forms must also be archived separately from the
    registry.

Phillips & Davis Best Current Practice [Page 75] RFC 5646 Language Tags September 2009

 o  Added text to Section 5 requiring IANA to send an announcement to
    an ietf-languages-announcements list whenever the registry is
    updated.
 o  Modification of the registry to use UTF-8 as its character
    encoding.  This also entails additional instructions to IANA and
    the Language Subtag Reviewer in the registration process.
 o  Modified the rules in Section 2.2.4 so that "exceptionally
    reserved" ISO 3166-1 codes other than 'UK' were included into the
    registry.  In particular, this allows the code 'EU' (European
    Union) to be used to form language tags or (more commonly) for
    applications that use the registry for region codes to reference
    this subtag.
 o  Modified the IANA considerations section (Section 5) to remove
    unnecessary normative [RFC2119] language.

9. References

9.1. Normative References

 [ISO15924]       International Organization for Standardization, "ISO
                  15924:2004.  Information and documentation -- Codes
                  for the representation of names of scripts",
                  January 2004.
 [ISO3166-1]      International Organization for Standardization, "ISO
                  3166-1:2006.  Codes for the representation of names
                  of countries and their subdivisions -- Part 1:
                  Country codes", November 2006.
 [ISO639-1]       International Organization for Standardization, "ISO
                  639-1:2002.  Codes for the representation of names
                  of languages -- Part 1: Alpha-2 code", July 2002.
 [ISO639-2]       International Organization for Standardization, "ISO
                  639-2:1998.  Codes for the representation of names
                  of languages -- Part 2: Alpha-3 code", October 1998.
 [ISO639-3]       International Organization for Standardization, "ISO
                  639-3:2007.  Codes for the representation of names
                  of languages - Part 3: Alpha-3 code for
                  comprehensive coverage of languages", February 2007.

Phillips & Davis Best Current Practice [Page 76] RFC 5646 Language Tags September 2009

 [ISO639-5]       International Organization for Standardization, "ISO
                  639-5:2008. Codes for the representation of names of
                  languages -- Part 5: Alpha-3 code for language
                  families and groups", May 2008.
 [ISO646]         International Organization for Standardization,
                  "ISO/IEC 646:1991, Information technology -- ISO
                  7-bit coded character set for information
                  interchange.", 1991.
 [RFC2026]        Bradner, S., "The Internet Standards Process --
                  Revision 3", BCP 9, RFC 2026, October 1996.
 [RFC2119]        Bradner, S., "Key words for use in RFCs to Indicate
                  Requirement Levels", BCP 14, RFC 2119, March 1997.
 [RFC2277]        Alvestrand, H., "IETF Policy on Character Sets and
                  Languages", BCP 18, RFC 2277, January 1998.
 [RFC3339]        Klyne, G., Ed. and C. Newman, "Date and Time on the
                  Internet: Timestamps", RFC 3339, July 2002.
 [RFC4647]        Phillips, A. and M. Davis, "Matching of Language
                  Tags", BCP 47, RFC 4647, September 2006.
 [RFC5226]        Narten, T. and H. Alvestrand, "Guidelines for
                  Writing an IANA Considerations Section in RFCs",
                  BCP 26, RFC 5226, May 2008.
 [RFC5234]        Crocker, D. and P. Overell, "Augmented BNF for
                  Syntax Specifications: ABNF", STD 68, RFC 5234,
                  January 2008.
 [SpecialCasing]  The Unicode Consoritum, "Unicode Character Database,
                  Special Casing Properties", March 2008, <http://
                  unicode.org/Public/UNIDATA/SpecialCasing.txt>.
 [UAX14]          Freitag, A., "Unicode Standard Annex #14: Line
                  Breaking Properties", August 2006,
                  <http://www.unicode.org/reports/tr14/>.
 [UN_M.49]        Statistics Division, United Nations, "Standard
                  Country or Area Codes for Statistical Use", Revision
                  4 (United Nations publication, Sales No. 98.XVII.9,
                  June 1999.

Phillips & Davis Best Current Practice [Page 77] RFC 5646 Language Tags September 2009

 [Unicode]        Unicode Consortium, "The Unicode Consortium. The
                  Unicode Standard, Version 5.0, (Boston, MA, Addison-
                  Wesley, 2003. ISBN 0-321-49081-0)", January 2007.

9.2. Informative References

 [CLDR]           "The Common Locale Data Repository Project",
                  <http://cldr.unicode.org>.
 [RFC1766]        Alvestrand, H., "Tags for the Identification of
                  Languages", RFC 1766, March 1995.
 [RFC2028]        Hovey, R. and S. Bradner, "The Organizations
                  Involved in the IETF Standards Process", BCP 11,
                  RFC 2028, October 1996.
 [RFC2046]        Freed, N. and N. Borenstein, "Multipurpose Internet
                  Mail Extensions (MIME) Part Two: Media Types",
                  RFC 2046, November 1996.
 [RFC2047]        Moore, K., "MIME (Multipurpose Internet Mail
                  Extensions) Part Three: Message Header Extensions
                  for Non-ASCII Text", RFC 2047, November 1996.
 [RFC2231]        Freed, N. and K. Moore, "MIME Parameter Value and
                  Encoded Word Extensions:
                  Character Sets, Languages, and Continuations",
                  RFC 2231, November 1997.
 [RFC2616]        Fielding, R., Gettys, J., Mogul, J., Frystyk, H.,
                  Masinter, L., Leach, P., and T. Berners-Lee,
                  "Hypertext Transfer Protocol -- HTTP/1.1", RFC 2616,
                  June 1999.
 [RFC2781]        Hoffman, P. and F. Yergeau, "UTF-16, an encoding of
                  ISO 10646", RFC 2781, February 2000.
 [RFC3066]        Alvestrand, H., "Tags for the Identification of
                  Languages", RFC 3066, January 2001.
 [RFC3282]        Alvestrand, H., "Content Language Headers",
                  RFC 3282, May 2002.
 [RFC3552]        Rescorla, E. and B. Korver, "Guidelines for Writing
                  RFC Text on Security Considerations", BCP 72,
                  RFC 3552, July 2003.

Phillips & Davis Best Current Practice [Page 78] RFC 5646 Language Tags September 2009

 [RFC3629]        Yergeau, F., "UTF-8, a transformation format of ISO
                  10646", STD 63, RFC 3629, November 2003.
 [RFC4645]        Ewell, D., "Initial Language Subtag Registry",
                  RFC 4645, September 2006.
 [RFC4646]        Phillips, A. and M. Davis, "Tags for Identifying
                  Languages", BCP 47, RFC 4646, September 2006.
 [RFC5645]        Ewell, D., Ed., "Update to the Language Subtag
                  Registry", September 2009.
 [UTS35]          Davis, M., "Unicode Technical Standard #35: Locale
                  Data Markup Language (LDML)", December 2007,
                  <http://www.unicode.org/reports/tr35/>.
 [iso639.prin]    ISO 639 Joint Advisory Committee, "ISO 639 Joint
                  Advisory Committee:  Working principles for ISO 639
                  maintenance", March 2000, <http://www.loc.gov/
                  standards/iso639-2/iso639jac_n3r.html>.
 [record-jar]     Raymond, E., "The Art of Unix Programming", 2003,
                  <urn:isbn:0-13-142901-9>.

Phillips & Davis Best Current Practice [Page 79] RFC 5646 Language Tags September 2009

Appendix A. Examples of Language Tags (Informative)

 Simple language subtag:
    de (German)
    fr (French)
    ja (Japanese)
    i-enochian (example of a grandfathered tag)
 Language subtag plus Script subtag:
    zh-Hant (Chinese written using the Traditional Chinese script)
    zh-Hans (Chinese written using the Simplified Chinese script)
    sr-Cyrl (Serbian written using the Cyrillic script)
    sr-Latn (Serbian written using the Latin script)
 Extended language subtags and their primary language subtag
 counterparts:
    zh-cmn-Hans-CN (Chinese, Mandarin, Simplified script, as used in
    China)
    cmn-Hans-CN (Mandarin Chinese, Simplified script, as used in
    China)
    zh-yue-HK (Chinese, Cantonese, as used in Hong Kong SAR)
    yue-HK (Cantonese Chinese, as used in Hong Kong SAR)
 Language-Script-Region:
    zh-Hans-CN (Chinese written using the Simplified script as used in
    mainland China)
    sr-Latn-RS (Serbian written using the Latin script as used in
    Serbia)

Phillips & Davis Best Current Practice [Page 80] RFC 5646 Language Tags September 2009

 Language-Variant:
    sl-rozaj (Resian dialect of Slovenian)
    sl-rozaj-biske (San Giorgio dialect of Resian dialect of
    Slovenian)
    sl-nedis (Nadiza dialect of Slovenian)
 Language-Region-Variant:
    de-CH-1901 (German as used in Switzerland using the 1901 variant
    [orthography])
    sl-IT-nedis (Slovenian as used in Italy, Nadiza dialect)
 Language-Script-Region-Variant:
    hy-Latn-IT-arevela (Eastern Armenian written in Latin script, as
    used in Italy)
 Language-Region:
    de-DE (German for Germany)
    en-US (English as used in the United States)
    es-419 (Spanish appropriate for the Latin America and Caribbean
    region using the UN region code)
 Private use subtags:
    de-CH-x-phonebk
    az-Arab-x-AZE-derbend
 Private use registry values:
    x-whatever (private use using the singleton 'x')
    qaa-Qaaa-QM-x-southern (all private tags)
    de-Qaaa (German, with a private script)
    sr-Latn-QM (Serbian, Latin script, private region)
    sr-Qaaa-RS (Serbian, private script, for Serbia)

Phillips & Davis Best Current Practice [Page 81] RFC 5646 Language Tags September 2009

 Tags that use extensions (examples ONLY -- extensions MUST be defined
 by revision or update to this document, or by RFC):
    en-US-u-islamcal
    zh-CN-a-myext-x-private
    en-a-myext-b-another
 Some Invalid Tags:
    de-419-DE (two region tags)
    a-DE (use of a single-character subtag in primary position; note
    that there are a few grandfathered tags that start with "i-" that
    are valid)
    ar-a-aaa-b-bbb-a-ccc (two extensions with same single-letter
    prefix)

Appendix B. Examples of Registration Forms

 LANGUAGE SUBTAG REGISTRATION FORM
 1. Name of requester: Han Steenwijk
 2. E-mail address of requester: han.steenwijk @ unipd.it
 3. Record Requested:
 Type:        variant
 Subtag:      biske
 Description: The San Giorgio dialect of Resian
 Description: The Bila dialect of Resian
 Prefix:      sl-rozaj
 Comments:    The dialect of San Giorgio/Bila is one of the
    four major local dialects of Resian
 4. Intended meaning of the subtag:
 The local variety of Resian as spoken in San Giorgio/Bila
 5. Reference to published description of the language (book or
 article):
  1. - Jan I.N. Baudouin de Courtenay - Opyt fonetiki rez'janskich

govorov, Varsava - Peterburg: Vende - Kozancikov, 1875.

Phillips & Davis Best Current Practice [Page 82] RFC 5646 Language Tags September 2009

 LANGUAGE SUBTAG REGISTRATION FORM
 1. Name of requester: Jaska Zedlik
 2. E-mail address of requester: jz53 @ zedlik.com
 3. Record Requested:
 Type:   variant
 Subtag: tarask
 Description: Belarusian in Taraskievica orthography
 Prefix: be
 Comments: The subtag represents Branislau Taraskievic's Belarusian
   orthography as published in "Bielaruski klasycny pravapis" by
   Juras Buslakou, Vincuk Viacorka, Zmicier Sanko, and Zmicier Sauka
   (Vilnia-Miensk 2005).
 4. Intended meaning of the subtag:
 The subtag is intended to represent the Belarusian orthography as
 published in "Bielaruski klasycny pravapis" by Juras Buslakou, Vincuk
 Viacorka, Zmicier Sanko, and Zmicier Sauka (Vilnia-Miensk 2005).
 5. Reference to published description of the language (book or
 article):
 Taraskievic, Branislau. Bielaruskaja gramatyka dla skol. Vilnia: Vyd.
 "Bielaruskaha kamitetu", 1929, 5th edition.
 Buslakou, Juras; Viacorka, Vincuk; Sanko, Zmicier; Sauka, Zmicier.
 Bielaruski klasycny pravapis. Vilnia-Miensk, 2005.
 6. Any other relevant information:
 Belarusian in Taraskievica orthography became widely used, especially
 in Belarusian-speaking Internet segment, but besides this some books
 and newspapers are also printed using this orthography of Belarusian.

Appendix C. Acknowledgements

 Any list of contributors is bound to be incomplete; please regard the
 following as only a selection from the group of people who have
 contributed to make this document what it is today.
 The contributors to RFC 4646, RFC 4647, RFC 3066, and RFC 1766, the
 precursors of this document, made enormous contributions directly or
 indirectly to this document and are generally responsible for the
 success of language tags.

Phillips & Davis Best Current Practice [Page 83] RFC 5646 Language Tags September 2009

 The following people contributed to this document:
 Stephane Bortzmeyer, Karen Broome, Peter Constable, John Cowan,
 Martin Duerst, Frank Ellerman, Doug Ewell, Deborah Garside, Marion
 Gunn, Alfred Hoenes, Kent Karlsson, Chris Newman, Randy Presuhn,
 Stephen Silver, Shawn Steele, and many, many others.
 Very special thanks must go to Harald Tveit Alvestrand, who
 originated RFCs 1766 and 3066, and without whom this document would
 not have been possible.
 Special thanks go to Michael Everson, who served as the Language Tag
 Reviewer for almost the entire RFC 1766/RFC 3066 period, as well as
 the Language Subtag Reviewer since the adoption of RFC 4646.
 Special thanks also go to Doug Ewell, for his production of the first
 complete subtag registry, his work to support and maintain new
 registrations, and his careful editorship of both RFC 4645 and
 [RFC5645].

Authors' Addresses

 Addison Phillips (editor)
 Lab126
 EMail: addison@inter-locale.com
 URI:   http://www.inter-locale.com
 Mark Davis (editor)
 Google
 EMail: markdavis@google.com

Phillips & Davis Best Current Practice [Page 84]

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