According to the structure, vowels can be divided into three types: monosyllabic vowels, complex vowels and nasal vowels. Monovowel vowels consist of monosyllables, including a, o, e, Mi, I, u, ü, -i[? ] (before I) and-I [ι] (after I), er and so on; The vowels of compound vowels are composed of compound vowels, including ai, ei, ao, ou, ia, ie, ua, uo, üe, iao, iou, uai and uei. Nasal vowels consist of vowels and nasal consonants. Mandarin * * * has sixteen nasal vowels, such as an, ian, uan, üan, en, in, uen, ün, ang, iang, uang, eng, ing, eng, ong and so on. Nasal vowels are also called nasal vowels.