Proto-Sino-Tibetan language

Proto-Sino-Tibetan (PST) is the hypothetical linguistic reconstruction of the Sino-Tibetan proto-language and the common ancestor of all languages in it, including the Sinitic languages, the Tibetic languages, Yi, Bai, Burmese, Karen, Tangut, and Naga. Paul K. Benedict (1972) placed a particular emphasis on Old Chinese, Classical Tibetan, Jingpho, Written Burmese, Garo, and Mizo in his discussion of Proto-Sino-Tibetan.

While Proto-Sino-Tibetan is commonly considered to have two direct descendants, Proto-Sinitic and Proto-Tibeto-Burman, in recent years several scholars have argued that this was not well-substantiated, and have taken to calling the group "Trans-Himalayan". In this case, Proto-Tibeto-Burman may be considered as equivalent to Proto-Sino-Tibetan if Sinitic is indeed not the first branch to split from Proto-Sino-Tibetan.

Features
Reconstructed features include prefixes such as the causative s-, the intransitive m-, the miscellaneous b-, d-, g-, and r-, suffixes -s, -t, and -n, and a set of conditioning factors that resulted in the development of tone in most languages of the family. The existence of such elaborate system of inflectional changes in Proto-Sino-Tibetan makes the language distinctive from some of its modern descendants, such as the Sinitic languages, which have mostly or completely become analytic.

Proto-Sino-Tibetan, like Old Chinese, also included numerous consonant clusters, and was not a tonal language.

Benedict (1972)
The table below shows consonant phonemes reconstructed by Benedict.

Peiros & Starostin (1996)
The reconstruction by Peiros & Starostin suggests a much more complex consonant inventory. The phonemes in brackets are reconstructions that are considered dubious.

Hill (2019)
The following tables show the reconstruction of Proto-Sino-Tibetan phonemes by Nathan Hill (2019).

The consonants can take coda position, as well as the cluster. While Hill does not reconstruct as an initial consonant due to Baxter and Sagart's Old Chinese reconstruction lacking such a phoneme, he mentions that Jacques and Schuessler suggest a  initial for some Old Chinese words due to potential Tibetan or Rgyalrongic cognates.

Hill also claims that his reconstruction is incomplete, as it does not account for Tibetic palatalization, proto-Burmish preglottalization, Sinitic aspirates, and the Sinitic type A and B distinction of syllables.

Final consonant changes
In Gong Huangcheng's reconstruction of the Proto-Sino-Tibetan language, the finals *-p, *-t, *-k, *-m, *-n, and *-ŋ in Proto-Sino-Tibetan remained in Proto-Sinitic and Proto-Tibeto-Burman. However, in Old Chinese, the finals *-k and *-ŋ that came after the close vowel *-i- underwent an irregular change of *-k＞*-t and *-ŋ ＞*-n. In Proto-Tibeto-Burman, *-kw and *-ŋw underwent a sound change to become *-k and *-ŋ respectively, while in Old Chinese those finals remained until Middle Chinese, where the finals underwent the same sound change.

Furthermore, in Proto-Tibeto-Burman, the finals *-g, *-gw, and *-d underwent the following changes:
 * 1) *-d＞*-y
 * 2) *-gw＞*-w
 * 3) *-g＞*-w when it follows the vowel *-u-
 * 4) *-g＞*-∅ when it follows the vowel *a and *-a-.

Vocabulary
Words which do not have reliable Sinitic parallels are accompanied by a (TB).