Editing ISO 639 (section)

===Three-letter code space===
Three-letter (formerly "Alpha-3") identifiers (for codes composed of 3 letters of the [[ISO basic Latin alphabet]]) are used in [[ISO 639-2|Set 2]], [[ISO 639-3|Set 3]], and [[ISO 639-5|Set 5]]. The number of languages and language groups that can be so represented is 26<sup>3</sup> = 17,576.

The common use of three-letter codes by three sets of ISO 639 requires some coordination within a larger system.

Set 2 defines four special codes <code>mis</code>, <code>mul</code>, <code>und</code>, <code>zxx</code>, a reserved range <code>qaa-qtz</code> (20 × 26 = 520 codes) and has 20 double entries (the B/T codes), plus 2 entries with deprecated B-codes.<!-- scc and scr --> This sums up to 520 + 22 + 4 = 546 codes that cannot be used in Set 3 to represent languages or in Set 5 to represent language families or groups.  The remainder is 17,576 – 546 = 17,030.
<!-- A further tighter upper limit can be calculated by subtracting the numbers of language collections from ISO 639-2. [Comment: This sentence appears in the article on ISO 639-3. If it is retained here, it might be rewritten to include in a broader overview of the Alpha-3 space. In general as I understand it, most 639-2 codes are also in 639-3, so the math here might be a little misleading.]-->

There are somewhere around six to seven thousand languages on Earth today.<ref>{{cite web|url=http://www.ethnologue.com/ethno_docs/distribution.asp?by=family |title=Statistical Summaries |publisher=Ethnologue |access-date=2012-08-05}}</ref> So those 17,030 codes are adequate to assign a unique code to each language, although some languages may end up with arbitrary codes that sound nothing like the traditional name(s) of that language.