The Proto-Tai language is not directly attested by any surviving texts, but has been reconstructed using the comparative method.
[2][3] The following table shows the consonants of Proto-Tai according to Li Fang-Kuei's A Handbook of Comparative Tai (1977), considered the standard reference in the field.
The table below lists the consonantal phonemes of Pittayawat Pittayaporn's reconstruction of Proto-Tai.
Li (and most other researchers) construct a Proto-Tai coda inventory that is identical with the system in modern Thai.
[2]: p. 193 Below is the consonantal syllabic coda inventory: Norquest (2021) reconstructs the voiceless retroflex stop /ʈ/ for Proto-Tai.
The subsequent reduction to monosyllables occurred independently in different branches, with the resulting apparent irregularities in synchronic languages reflecting Proto-Tai sesquisyllables.
Gedney (1972) also included a list of diagnostic words to determine tonal values, splits, and mergers for particular Tai languages.
Unlike its modern-day monosyllabic descendants, Proto-Tai was a sesquisyllabic language (Pittayaporn 2009).
[2]: p. 64 Legend: During the evolution from Proto-Tai to modern Tai languages, monosyllabification involved a series of five steps.
[18] Proto-Tai had a SVO (subject–verb–object) word order like Chinese and almost all modern Tai languages.
Examples of Kra-Hlai-Tai isoglosses as identified by Norquest (2021):[5] Examples of Hlai-Be-Tai isoglosses as identified by Norquest (2021):[5] Examples of Be-Tai isoglosses as identified by Norquest (2021):[5] Ostapirat (2023) notes that as in Proto-Hmong–Mien, prenasalized consonant initials in Proto-Tai often correspond with prenasalized consonant initials in Old Chinese (with the Old Chinese reconstructions below from Baxter & Sagart 2014[19]).