Actually, it was made up in the past. Because there was no Chinese Pinyin in the old days, actors in traditional Chinese opera and Quyi all used the same vowel characters as the rhyming standard, and such characters had 13 groups, so the so-called "Thirteen Avenue" was formed. Later, the Quyi circle summed up a sentence: "Xiaomei slipped out of the room to sit in the east, west, north and south" (no textual research) Jiang Yang-Ang Yan Qian-An Yichen-I am pregnant-Ai Gusu -U ashes -UI, ei Middle East-ong, eng, ing Thank you -IE You Qiu-Ou Fahua-A. There are also two small sayings: "The fine print is the day before yesterday, and the villain is a child." Xiao yanqian 'er: including a, ua, ia, ai, uai, an, uan, üan and ian (namely, fahua, Huailai and Yan Qian). Little people: including vowels such as o, e, i, u, ü, ie, üe, ei, uei(ui), en, in, uen(un), ün, uo and er.