Distribution of the Southwestern Tai languages.
The Southwestern Tai, Southwestern Thai or Thai languages are an established branch of the Tai languages of Southeast Asia. They include Siamese (Central Thai), Lanna (Northern Thai), Lao, Isan, Shan and others.
The internal classification of the Southwestern Tai languages is still not well agreed on.
Chamberlain (1975) divides Southwestern Tai into 4 branches.
Chamberlain based his classification on the following phonological patterns. (Note: For an explanation of the notation system for Tai tones, see Proto-Tai language#Tones.)
The Tai Muong Vat of Yen Chau, Vietnam is a PH-type language like Lao, even though it is geographically surrounded by Black Tai (Theraphan 2003; Chamberlain 1984).
Edmondson & Solnit (1997) 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 (including Tai Mau) 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" (1994). The following features set off the Nüa-Khamti group from all the other Southwestern Tai languages.
Luo Yongxian (2001) also recognizes the uniqueness of Dehong Tai (Tai Nuea), 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.
See also: Tai languages § Pittayaporn (2009)
According to Pittayaporn (2009:301), Southwestern Tai (his subgroup Q) is defined by a phonological shift of *kr- → *ʰr-.
Pittayaporn (2014) also suggests that Southwestern Tai began to disperse southward after the 7th century C.E. but before the 11th century C.E. (between 700 and 1000 C.E., during the late Tang dynasty or early Song dynasty), as evidenced by loanwords from Late Middle Chinese.
Pittayaporn (2018) 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.
Pittayaporn, et al. (2018) note that following sound changes from Proto-Southwestern Tai (PSWT) 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.
Southern Thai (Pak 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 (Central–Eastern) 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 (China), Pu Ko (Laos), Pa Di (China), Tai Thanh (Vietnam), Tai Long (Laos), Tai Hongjin (China), Yong (Thailand). It is not clear where they belong in the classification above. Ethnologue also lists under Tai, without further classification, Kuan (Laos), Tai Do (Viet Nam), Tai Pao (Laos), and Tay Khang (Laos). Geographically these would all appear to be Southwestern.
Ethnologue also includes Tày Sa Pa (Sapa) 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.