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International standard for three letter codes identifying languages
ISO 639 3:2007, Codes for the representation of names of languages – Part 3: Alpha 3 code for comprehensive coverage of languages, is an international standard for language codes in the ISO 639 series. It defines three letter codes for identifying languages. The standard was published by International Organization for Standardization (ISO) on 1 February 2007. ISO 639 3 extends the ISO 639 2 alpha 3 codes with an aim to cover all known natural languages. The extended language coverage was based primarily on the language codes used in the Ethnologue (volumes 10–14) published by SIL International, which is now the registration authority for ISO 639 3. It provides an enumeration of languages as complete as possible, including living and extinct, ancient and constructed, major and minor, written and unwritten. ISO 639 3 is intended for use as metadata codes in a wide range of applications. It is widely used in computer and information systems, such as the Internet, in which many languages need to be supported. In archives and other information storage, it is used in cataloging systems, indicating what language a resource is in or about. The codes are also frequently used in the linguistic literature and elsewhere to compensate for the fact that language names may be obscure or ambiguous. Find a language Enter an ISO 639 3 code to find the corresponding language article.

Since the code is three letter alphabetic, one upper bound for the number of languages that can be represented is 26 × 26 × 26 = 17,576. Since ISO 639 2 defines special codes (4), a reserved range (520) and B only codes (22), 546 codes cannot be used in part 3. Therefore, a stricter upper bound is 17,576 − 546 = 17,030. The upper bound gets even stricter if one subtracts the language collections defined in 639 2 and the ones yet to be defined in ISO 639 5.

"A collective language code element is an identifier that represents a group of individual languages that are not deemed to be one language in any usage context." These codes do not precisely represent a particular language or macrolanguage. While ISO 639 2 includes three letter identifiers for collective languages, these codes are excluded from ISO 639 3. Hence ISO 639 3 is not a superset of ISO 639 2. ISO 639 5 defines 3 letter collective codes for language families and groups, including the collective language codes from ISO 639 2.

Four codes are set aside in ISO 639 2 and ISO 639 3 for cases where none of the specific codes are appropriate. These are intended primarily for applications like databases where an ISO code is required regardless of whether one exists. (uncoded languages, originally an abbreviation for 'miscellaneous') is intended for languages which have not (yet) been included in the ISO standard. (multiple languages) is intended for cases where the data includes more than one language, and (for example) the database requires a single ISO code. (undetermined) is intended for cases where the language in the data has not been identified, such as when it is mislabeled or never had been labeled. It is not intended for cases such as Trojan where an unattested language has been given a name. (no linguistic content / not applicable) is intended for data which is not a language at all, such as animal calls. In addition, 520 codes in the range – are 'reserved for local use'. For example, Rebecca Bettencourt assigns a code to constructed languages, and new assignments are made upon request. The Linguist List uses them for extinct languages. Linguist List has assigned one of them a generic value: , unnamed proto language. This is used for proposed intermediate nodes in a family tree that have no name.