© 2020, University of Hawaii Press. All rights reserved. Tai Ahom (Southwestern Tai) mostly survives in manuscripts. (Terwiel 1988; Morey 2015). It has long been held that Ahom retained many archaic features lost in most modern Tai languages. For example, Li (1977: 87-89) reconstructs the cluster *phr-as evidence from Ahom in words like phra ‘rock’ and phrai ‘walk’. However, Diller (1992), argues that Ahom exhibited the “pan-Tai consonant mergers of the sort in which the sounds of the “low series consonants presumably fell together with certain of the others” and hence is not archaic. In order to uncover the true nature of Ahom, this paper investigates how each of Ahom graph relates to the reconstructed phonemes in Proto-Southwestern Tai (Li 1977; Pittayaporn 2009). Our analysis was based on eight carefully analyzed manuscripts, identified allographic variations, suggesting mergers and retentions of Tai phonemic contrasts are common among the modern Shan varieties and the lack of archaic features claimed by earlier authors.