*Branch with distinguishing innovation: *BCD 123-4
**Siamese
**Phu Tai
**Lao Neua spoken at "Nam Tha"
**Phuan spoken at Ban Mi, Lopburi, Thailand and Pak Seng
*Branch with distinguishing innovation: *BCD 1-23-4; B≠DL
**Lao
**Southern Thai
The Tai Muong Vat of Yen Chau, Vietnam is a PH-type language like Lao, even though it is geographically surrounded by Black Tai.
Edmondson & Solnit (1997)
Edmondson & Solnit divide the Southwestern Tai languages into two major subgroups. According to this classification, Dehong Tai and Khamti are the first languages to have split off from the Southwestern Tai branch.
A transition zone between the Northern and Southern groups occurs among the Tai languages around the Burma-China border region of Mangshi, Namhkam, and Mu-se near Ruili. This bipartite division of Southwestern Tai is argued for by Edward Robinson in his paper "Features of Proto-Nüa-Khamti". The following features set off the Nüa-Khamti group from all the other Southwestern Tai languages.
Labialized velar stops have become velar stops.
Tripartite split of the A tone A1-23-4
Merger of A23 and B4
The low vowels /ɛ/ and /ɔ/ have merged with /e/ and /o/, respectively.
*ʔb > m
Luo (2001)
Luo Yongxian also recognizes the uniqueness of Dehong Tai, but argues for that it should be placed in a separate Northwestern Tai branch with Southwestern Tai as a sister branch. Luo claims that the Northwestern Tai branch has many Northern Tai and Central Tai features that are not found in Southwestern Tai. His proposed tree for the Tai branch is as follows.
Tai
*Northern
*Central
*Southwestern
*Northwestern
Pittayaporn (2009)
According to Pittayaporn, Southwestern Tai is defined by a phonological shift of *kr- → *ʰr-. Pittayaporn also suggests that Southwestern Tai began to disperse southward after the 7th century C.E. but before the 11th century C.E., as evidenced by loanwords from Late Middle Chinese. Pittayaporn recognizes two branches within Southwestern Tai, namely Eastern and Western. The Eastern branch consists of the closely related languages Black Tai, White Tai, and Red Tai, while the Western branch is much more internally diverse. The Western branch also contains a Southern group consisting of Thai and Lao. ;Southwestern Tai
Eastern branch: Black Tai, White Tai, Red Tai
Western branch: Shan varieties, Lue, Yuan, Lao, Thai
*Southern sub-branch: Thai, Lao, etc.
Pittayaporn, et al. note that following sound changes from Proto-Southwestern Tai to the Tai varieties represented in the Sukhothai and Ayutthaya inscriptions, and conclude that the Sukhothai and Ayutthaya inscriptions in fact represent the same language.
the merger of dorsal obstruents
the merger of PSWT *aɯ and *aj
the merger of PSWT *ɲ-, *j- and *ʔj-
the loss of voicing distinction in sonorants
*ɓl- > d-
*kʰr- > kʰ-
*ʰr- > h-
Languages
Southern Thai is often posited to be the most divergent; it seems to retain regular reflexes of early tonal developments that were obscured in the other languages. The reconstructed language is called Proto-Thai; cf. Proto-Tai, which is the ancestor of all of the Tai languages. The following tree follows that of Ethnologue
According to Ethnologue, other Southwestern languages are Tai Ya, Pu Ko, Pa Di, Tai Thanh, Tai Long, Tai Hongjin, Yong. It is not clear where they belong in the classification above. Ethnologue also lists under Tai, without further classification, Kuan, Tai Do, Tai Pao, and Tay Khang. Geographically these would all appear to be Southwestern. Ethnologue also includes Tày Sa Pa of Vietnam, which Pittayaporn excludes from Southwestern Tai but classifies as the most closely related language outside of that group. Pittayaporn also includes Yoy, which Ethnologue classifies as a Northern Tai language.
