What were the four divisions of Middle Chinese?

Determining the nature of the four Divisions of the Qièyùn is a fundamental problem in the study of the phonetic history of Chinese. Analyses by Pulleyblank and Baxter make it possible to bring out two major changes from Old Chinese to Middle Chinese: a two-way split of the vowel system, and later the lenition of medial -r-. The use of models drawn from Mon-Khmer voice type register languages made it possible to reconstruct the phonetic bases of the four divisions. Div. I groups tense rimes, Div. II groups velarized rimes resulting from medial -r-, Div. III groups lax/breathy rimes. As for Div. IV, it represents rimes with the diphthong ie.