Thai language
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), however, many Thai dialects have only one such short–long pair (/a aː/), and in general it is difficult or impossible to find minimal short–long pairs in Thai that involve vowels other than /a/ and where both members have frequent correspondences throughout the Tai languages. More specifically, he notes the following facts about Thai:

Furthermore, the vowel that corresponds to short Thai /a/ has a different and often higher quality in many of the Tai languages compared with the vowel corresponding to Thai /aː/.

This leads Li to posit the following:

Not all researchers agree with Li. Pittayaporn (2009), for example, reconstructs a similar system for Proto-Southwestern-Tai, but believes that there was also a mid back unrounded vowel /ə/ (which he describes as /ɤ/), occurring only before final velar /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.
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