ISO 639 is a standard by the International Organization for Standardization (ISO) concerned with representation of languages and language groups. It currently consists of four sets (1-3, 5) of code, named after each part which formerly described respective set (part 4 was guidelines without its own coding system); a part 6 was published but withdrawn. It was first approved in 1967 as a single-part ISO Recommendation, ISO/R 639,^[1] superseded in 2002 by part 1 of the new series, ISO 639-1,^[2] followed by additional parts. All existing parts of the series were consolidated into a single standard in 2023,^[3] largely based on the text of ISO 639-4.

The language codes defined in the several sections of ISO 639 are used for bibliographic purposes and, in computing and internet environments, as a key element of locale data. The codes also find use in various applications, such as Wikipedia URLs for its different language editions.

Each set of the standard is maintained by a maintenance agency, which adds codes and changes the status of codes when needed. ISO 639-6 was withdrawn in 2014,^[8] and not included in ISO 639:2023.

Scopes:

• Individual languages
• Macrolanguages (Set 3)
• Collections of languages (Sets 1, 2, 5). Some collections were already in Set 2, and others were added only in Set 5:
  • Remainder groups: 36 collections in both Set 2 and 5 are of this kind — for compatibility with Set 2 when Set 5 was still not published, the remainder groups do not contain any language and collection that was already coded in Set 2 (however new applications compatible with Set 5 may treat these groups inclusively, as long they respect the containment hierarchy published in Set 5 and they use the most specific collection when grouping languages);
    • The only collection which previously assigned with two-alphabet code is Bihari (bh) during the Part 1 era, which deprecated in June 2021.^[9]
  • Regular groups: 29 collections in both Sets 2 and 5 are of this kind — for compatibility with Set 2, they can not contain other groups;
  • Families: 50 new collections coded only in Set 5 (including one containing a regular group already coded in Set 2) — for compatibility with Set 2, they may contain other collections except remainder groups.
• Dialects: they were intended to be covered by former ISO 639-6 (proposed but now withdrawn).
• Special situations (Sets 2, 3).
• Reserved for local use (Sets 2, 3). Also used sometimes in applications needing an two-letter code like standard codes in Sets 1 and 2 (where the special code mis is not suitable), or a three-letter code for collections like standard codes in Set 5.

Types (for individual languages):^{[needs update]}

• Living languages (Sets 2, 3) (all macrolanguages are living languages)^[10]
• Extinct languages (Sets 2, 3) (599,^[11] 5 of them are in Set 2: chb, chg, cop, lui, sam; none are in Set 1)
• Ancient languages (Sets 1, 2, 3) (124,^[12] 19 of them are in Set 2; and 5 of them, namely ave, chu, lat, pli and san, also have a code in Set 1: ae, cu, la, pi, sa)
• Historical languages (Sets 2, 3) (89,^[13] 16 of them are in Set 2; none are in Set 1)
• Constructed languages (Sets 1, 2, 3) (23,^[14] 9 of them in Set 2: afh, epo, ido, ile, ina, jbo, tlh, vol, zbl; 5 of them in Set 1: eo, ia, ie, io, vo)

Individual languages and macrolanguages with two distinct three-letter codes in Set 2:

• Bibliographic (some of them were deprecated, none were defined in Set 3): these are legacy codes (based on language names in English).
• Terminologic (also defined in Set 3): these are the preferred codes (based on native language names, romanized if needed).
• All others (including collections of languages and special/reserved codes) only have a single three-letter code for both uses.

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The different sets of ISO 639 are designed to work together, in such a way that no code means one thing in one set and something else in another. However, not all languages are in all sets, and there is a variety of different ways that specific languages and other elements are treated in the different sets. This depends, for example, whether a language is listed in Sets 1 or 2, whether it has separate B/T codes in Set 2, or is classified as a macrolanguage in Set 3, and so forth.

These various treatments are detailed in the following chart. In each group of rows (one for each scope of Set 3), the last four columns contain codes for a representative language that exemplifies a specific type of relation between the sets of ISO 639, the second column provides an explanation of the relationship, and the first column indicates the number of elements that have that type of relationship. For example, there are four elements that have a code in Set 1, have a B/T code, and are classified as macrolanguages in Set 3. One representative of these four elements is "Persian" fa/per/fas.

These differences are due to the following factors.

In ISO 639 Set 2, two distinct codes were assigned to 22 individual languages, namely a bibliographic and a terminology code (B/T codes).^[15] B codes were included for historical reasons because previous widely used bibliographic systems used language codes based on the English name for the language. In contrast, the Set 1 codes were based on the native name for the language, and there was also a strong desire to have Set 2 codes (T codes) for these languages which were similar to the corresponding 2-character code in Set 1.

• For instance, the German language (Set 1: de) has two codes in Set 2: ger (B code) and deu (T code), whereas there is only one code in Set 2, eng, for the English language.
• 2 former B codes were withdrawn, leaving today only 20 pairs of B/T codes.

Individual languages in Set 2 always have a code in Set 3 (only the Set 2 terminology code is reused there) but may or may not have a code in Set 1, as illustrated by the following examples:

• Set 3 eng corresponds to Set 2 eng and Set 1 en
• Set 3 ast corresponds to Set 2 ast but lacks a code in Set 1.

Some codes (62) in Set 3 are macrolanguages. These are groups containing multiple individual languages that have a good mutual understanding and are commonly mixed or confused. Some macrolanguages developed a default standard form on one of their individual languages (e.g. Mandarin is implied by default for the Chinese macrolanguage, other individual languages may be still distinguished if needed but the specific code cmn for Mandarin is rarely used).

• 1 macrolanguage has a Set 2 code and a Set 1 code, while its member individual languages also have codes in Set 1 and Set 2: nor/no contains non/nn, nob/nb; or
• 4 macrolanguages have two Set 2 codes (B/T) and a Set 1 code: per/fas/fa, may/msa/ms, alb/sqi/sq, and chi/zho/zh;
• 28 macrolanguages have a Set 2 code but no Set 1 code;
• 29 other macrolanguages only have codes in Set 3.

Collective codes in Set 2 have a code in Set 5: e.g. aus in Sets 2 and 5, which stands for Australian languages.

• Some codes were added in Set 5 but had no code in Set 2: e.g. sqj

Sets 2 and 3 also have a reserved range and four special codes:

• Codes qaa through qtz are reserved for local use.
• There are four special codes: mis for languages that have no code yet assigned, mul for "multiple languages", und for "undefined", and zxx for "no linguistic content, not applicable".

• IETF language tags (based on ISO 639)
• ISO 3166 (codes for countries)
• ISO 15924 (codes for writing systems)
• Codes for constructed languages
• Language code
• Language families and languages
• List of languages
• List of official languages
• Lists of ISO 639 codes