The aim of the present study of /thîi/ is three-fold. The first is to study the various occurrences of /thîi/ in modern Thai so as to identify its various functions. The second is to investigate the historical development of /thîi/ in four periods: Sukhothai, Ayutthaya, Mid-Ratanakosin covering the reigns of King Rama IV and V, and the present day Thai, henceforth, modern Thai. This leads to the third aim, that is to see whether the historical development agrees with the grammaticalization pathway of /thîi/ proposed by W. Bisang in his important paper on processes of grammaticalization of nouns and verbs in east and mainland South East Asian languages (Bisang, 1996). The last section of the paper surveys the use of equivalent words of /thîi/ in two other Tai languages.