确定《切韵》四等的性质是汉语语音史上的根本问题。蒲立本跟白一平的分析勾勒出从上古到中古汉语的两次重大演变:元音系统的两分以及后来的介音-r-的弱化。参照孟高棉语族发声分域语言的演变模型,我们得以重构四等的语音基础:一等韵是紧韵母,二等韵是介音-r-变来的软腭化韵母,三等韵是松/气声韵母,四等韵则是带复元音ie的韵母。 It is the fundamental problem in the history of Chinese phonetics that the nature of the fourth class of “Qie Yun” is determined. The analysis of Pu Li-Ben and Bai Yi-Ping outlines two major evolutions from Middle Ages to Middle Ages: the dichotomy of the vowel system and the subsequent weakening of the interrogative-r-. We can reconstruct the fourth-class phonetic foundation by referring to the evolvement model of phonetic sublanguage in Meng Khmer language family: the first rhyme is the tense verb, the second rhyme is the utterance-r-the soft palate vowel, the third rhyme is Loose / qi vowels, four rhyme is vowel vowels with complex vowels.