Vowel developments
The vowel system of modern Thai contains nine pure vowels and three centering diphthongs, each of which can occur short or long. According to Li (1977[full citation needed]), however, many Thai dialects have only one such short–long pair (/a aː/), and in general it is difficult or impossible to find minimal short–long pairs in Thai that involve vowels other than /a/ and where both members have frequent correspondences throughout the Tai languages. More specifically, he notes the following facts about Thai: