Languages can be divided into three types with respect to the encoding of comitatives and instrumentals: identity, differentiation and mixed (Stolz, Stroh and Urdze 2013). Diachronic data from Thai dating from 13th to 21st centuries ce suggests that these three language types correspond to the three stages of development of the relation between the two categories in Thai, which progress as follows. Between the 13th and the mid-18th centuries, Thai employed the pattern of identity. In general, the preposition dûaj ‘with’ was the relator for the two categories. Later, in the late 18th and early 19th centuries, Thai displayed mixed structures. While the preposition dûaj was preserved to encode both categories, the preposition kàp ‘and/with’ was also the relator for the comitative. Still later, from the mid-19th century to the present, Thai favored differentiation. The preposition dûaj remains in its function as an instrumental relator, while the preposition kàp has been employed as a comitative relator.