Yüan opera, if we limit our material to the one hundred examples found in the Yüan-ch'ü hsüan (YCH), seems to be a well defined genre, the numerous formal peculiarities of which can be shown to have evolved from earlier entertainment forms in a manner truly comforting to the literary historian. Analyzed, Yiian drama shows a highly consistent structure with the following features:
1. Four “acts” with the option of a demi-act, “wedge,” which can appear between any two acts, or at the beginning of the tsa-chü (Yüan opera).
2. Within each act all arias were sung to tunes of the same mode.