Qu (曲) is a literary form that came into being later than poetry and ci (词). It generally refers to the northern- and southern-style melodies created in the Song and Jin dynasties. Northern melodies were composed mostly with tunes in northern China and performed in northern dialect, while southern melodies had southern tunes and southern dialect. Since qu reached its peak in the Yuan Dynasty, it is generally known as Yuan qu or Yuan opera. Qu is similar to ci in form but is more flexible in sentence structure, and colloquial language is used. There are two main types of qu: one is northern zaju (杂剧) opera and southern chuanqi (传奇) opera; such qu is known as xiqu (戏曲) or juqu (剧曲). The other type is sanqu (散曲) or lyric songs, also known as qingqu (清曲). As with other forms of poetry, sanqu describes a scene, a sentiment or an event and can be sung, but it has no spoken parts or instructions for performers’ movements and expressions. Generally speaking, the old-style opera is much more accomplished and influential than sanqu. The Yuan period was a golden age in the development of Chinese opera. There are more than 80 known playwrights from that time. Guan Hanqing, Bai Pu, Ma Zhiyuan, and Zheng Guangzu represent different styles from different stages of the Yuan opera, and they are recognized as the four leading Yuan opera writers. Yuan opera has distinctive plots and artistic appeal. Together with Tang and Song poetry and Ming and Qing fiction, it marks an important milestone in the historical development of Chinese literature.
When it comes to leading opera writers of the Yuan Dynasty, Guan Hanqing, Zheng Guangzu, Bai Pu, and Ma Zhiyuan come to mind. (Wang Jide: On the Melody and Writing of Chinese Operas)