Although a small number of Six Dynasties heptasyllabic quatrains are extant, and Early Tang poets experimented with the form, stylistically mature qijue poetry was an invention of the High Tang poets, most notably Wang Changling and Li Bai. Qijue developed along with Tang popular music, for which it was the major song form. Thus initially the thematic scope was narrow: qijue lyrics were generally limited to popular yuefu themes and those describing parting from friends and loved ones. Only gradually did the scope of qijue themes expand, until by the Middle and Late Tang, the form had become a flexible tool for personal expression.
Egan, C., & Cai, Z. (2022, Sep 6). The Tang Dynasty: Quatrains: The boudoir and the frontier: High Tang . In How to read Chinese poetry podcast. Hong Kong: Lingnan University. Retrieved from https://commons.ln.edu.hk/poetry_podcast/32/