What kind of poetic form is line?

The fusion of ancient poetry. After the Han and Wei Dynasties, there were quite a few Yuefu poems with the theme of "Song" and "Xing". Although the names are different, there is no strict difference between them. After that, there was a "singing line". Its syllables and rhythms are generally free, and its form adopts the ancient style of five words, seven words and miscellaneous words, which is full of changes. "Line" means music. See Biography of Historical Records of Sima Xiangru, Sima Zhen and Suoyin.