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
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 (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
4.6. Considerations for Private Use Subtags . . . . . . . . . . 685. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 695.1. Language Subtag Registry . . . . . . . . . . . . . . . . . 695.2. Extensions Registry . . . . . . . . . . . . . . . . . . . 716. Security Considerations . . . . . . . . . . . . . . . . . . . 717. Character Set Considerations . . . . . . . . . . . . . . . . . 728. Changes from RFC 4646 . . . . . . . . . . . . . . . . . . . . 739. References . . . . . . . . . . . . . . . . . . . . . . . . . . 769.1. Normative References . . . . . . . . . . . . . . . . . . . 769.2. Informative References . . . . . . . . . . . . . . . . . . 78
Appendix A. Examples of Language Tags (Informative) . . . . . . . 80
Appendix B. Examples of Registration Forms . . . . . . . . . . . 82
Appendix C. Acknowledgements . . . . . . . . . . . . . . . . . . 831. 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
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
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
form tags. It also defines a mechanism for private use values and
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.
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
langtag = language
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
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
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
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
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
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'
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
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
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
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
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
solely by private agreement and is not defined by the rules in
this section or in any standard or registry defined in this
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
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
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
A. "ISO 639-2:1998 - Codes for the representation of names of
languages -- Part 2: Alpha-3 code - edition 1" [ISO639-2]
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
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
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-
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
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
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
"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
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
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
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.
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
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
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
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.
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
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
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
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.
"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
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
1. Variant subtags that begin with a letter (a-z, A-Z) MUST be
at least five characters long.
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
* 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"
"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
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
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
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
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.
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
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
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.
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
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