It is taken for granted in Chinese linguistics that the medial glide belongs to the rhyme (or final), and that the glides-i-and-u-exhibit the same behavior-they both belong to the rhyme. Insight into the constituency of the medial glides comes from phonological processes that target sub-syllabic strings. In this paper, I show that the medial glides in Middle Chinese exhibit asymmetric behavior. Specifically, -i-belongs to the rhyme, but-u-does not. Their syllabic position is as follows: Cu[iVC]. Supportive evidence comes from the fanqie entries recorded in the Qieyun (c. 600(subscript A. D.)). The same asymmetry is attested in modern Mandarin, especially in the dialects spoken in Shanxi Province. The 'split personality' of the medial glides in Middle Chinese proves remarkably conservative.