Length contrast of high vowels in the Thai language of Sukhothai period: What do the inscriptions say?

In an attempt to study the length distinction of high vowels in Sukhothai Thai, this research compares an analysis of the graphemic system and spelling variations found in the Sukhothai inscriptions with the phonemes in Proto-Southwestern Tai (PSWT) and donor languages of the loanwords. The result indicates that short and long high vowels in PSWT behave differently in phonemic-graphemic mapping. Short vowels are mapped with ⟨i⟩ and ⟨u⟩ whereas long vowels with ⟨ī⟩, ⟨ï̄⟩, and ⟨ū⟩. In addition, the existing spelling variations are limited to specific kinds of words, namely: open-syllable words, loanwords, and function words, all of which are susceptible to variation in spelling. These findings attest to the existence of length contrast in Sukhothai Thai.