Editing Thai language (section)

=== Vowel developments ===
The vowel system of modern Thai contains nine pure vowels and three centering diphthongs, each of which can occur short or long. According to Li (1977{{Full citation needed|date=November 2012}}), however, many Thai dialects have only one such short–long pair ({{IPA|/a aː/}}), and in general it is difficult or impossible to find minimal short–long pairs in Thai that involve vowels other than {{IPA|/a/}} and where both members have frequent correspondences throughout the Tai languages.  More specifically, he notes the following facts about Thai:
*In [[open syllable]]s, only long vowels occur. (This assumes that all apparent cases of short open syllables are better described as ending in a glottal stop.  This makes sense from the lack of tonal distinctions in such syllables, and the glottal stop is also reconstructible across the Tai languages.)
*In [[closed syllable]]s, the long high vowels {{IPA|/iː ɯː uː/}} are rare, and cases that do exist typically have diphthongs in other Tai languages.
*In closed syllables, both short and long mid {{IPA|/e eː o oː/}} and low {{IPA|/ɛ ɛː ɔ ɔː/}} do occur.  However, generally, only words with short {{IPA|/e o/}} and long {{IPA|/ɛː ɔː/}} are reconstructible back to Proto-Tai.
*Both of the mid back unrounded vowels {{IPA|/ɤ ɤː/}} are rare, and words with such sounds generally cannot be reconstructed back to Proto-Tai.
Furthermore, the vowel that corresponds to short Thai {{IPA|/a/}} has a different and often higher quality in many of the Tai languages compared with the vowel corresponding to Thai {{IPA|/aː/}}.

This leads Li to posit the following:
#Proto-Tai had a system of nine pure vowels with no length distinction, and possessing approximately the same qualities as in modern Thai: high {{IPA|/i ɯ u/}}, mid {{IPA|/e ɤ o/}}, low {{IPA|/ɛ a ɔ/}}.
#All Proto-Tai vowels were lengthened in open syllables, and low vowels were also lengthened in closed syllables.
#Modern Thai largely preserved the original lengths and qualities, but lowered {{IPA|/ɤ/}} to {{IPA|/a/}}, which became short {{IPA|/a/}} in closed syllables and created a phonemic length distinction {{IPA|/a aː/}}.  Eventually, length in all other vowels became phonemic as well and a new {{IPA|/ɤ/}} (both short and long) was introduced, through a combination of borrowing and sound change.  Li believes that the development of long {{IPA|/iː ɯː uː/}} from diphthongs, and the lowering of {{IPA|/ɤ/}} to {{IPA|/a/}} to create a length distinction {{IPA|/a aː/}}, had occurred by the time of Proto-Southwestern-Tai, but the other missing modern Thai vowels had not yet developed.

Not all researchers agree with Li. Pittayaporn (2009{{Full citation needed|date=November 2012}}), for example, reconstructs a similar system for Proto-Southwestern-Tai, but believes that there was also a mid back unrounded vowel {{IPA|/ə/}} (which he describes as {{IPA|/ɤ/}}), occurring only before final velar {{IPA|/k ŋ/}}.  He also seems to believe that the Proto-Southwestern-Tai vowel length distinctions can be reconstructed back to similar distinctions in Proto-Tai.