This bipartite division of Southwestern Tai is argued for by Edward Robinson in his paper "Features of Proto-Nüa-Khamti" (1994).

According to Pittayaporn (2009:301), Southwestern Tai (his subgroup Q) is defined by a phonological shift of *kr- → *ʰr-.

[6] Pittayaporn (2014) also suggests that Southwestern Tai began to disperse southward after the 7th century C.E.

Pittayaporn, et al. (2018)[9] 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.

[11] 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.