Each qupai has a fixed melody and a set requirement for the number of characters.
Tips
history
Qupai are the melodic templates of Yuan-dynasty sanqu (散曲) and later Kunqu opera. Like 词牌 in ci poetry, the title fixes the tune, rhyme scheme, and tonal pattern while the words vary.
usage
Compare 词牌 (ci tune-pattern, Song dynasty) and 曲牌 (qu tune-pattern, Yuan and later). Both work the same way: pick a pattern, fill in new lyrics.