Understanding Tone Change: In Search of Stability-Induced Tonal Fusion in Southwestern Tai Languages

Abstract

Xishuangbanna Tai Lue (China) features six lexical tones and six emerging grammatical tones. Each grammatical tone arises through the fusion of two lexical tones, a process driven by tonal stability mechanism following segmental deletion. Among Southwestern Tai languages, only Tai Phake and Central Thai have also been reported to exhibit tonal stability behavior. Drawing primarily on fieldwork data, this paper outlines the principal properties of grammatical tones in Xishuangbanna Tai Lue and investigates why, in identical morphosyntactic contexts, stability-induced tonal fusion does not occur in Tai Phake, Central Thai, Northern Thai, and Lampang Tai Lue (Thailand). This absence is attributable to factors including word shape, differentiation of tonal functions, and language contact. Overall, the findings contribute to the documentation of emerging grammatical tone systems and advance our understanding of tone change by addressing the actuation problem—namely, why a given change occurs in one language but not in others, despite comparable structural conditions.