清代小學家江永：「音韻有四等，一等洪大，二等次大，三四皆細，而四尤細」。宜家普遍認為韻母嘅分等係對韻母中介音嘅有無同韻腹（字嘅主要元音）開口度大小和發音部位前後嘅分析。王力學派認為：一、二等無介音，三、四等有介音；一等韻韻腹的發音部位偏後（如模韻、歌韻），二、四等多偏前（如麻韻、先韻）。三等韻中有啲就係帶介音嘅一等韻（如東韻的一、三等），有啲係相對於四等韻而言韻腹爲開口度較大的前元音（如王力認為三等嘅仙韻同四等嘅先韻都帶介音，然而仙嘅韻腹爲 ，先嘅韻腹爲，前者的開口度大於後者）。
Jiang Yong, a scholar in the Qing Dynasty, said: "There are four levels of rhyme, the first class is large, the second class is large, the third and fourth are small, and the fourth is especially small." It is now generally accepted that the classification of finals is an analysis of the presence or absence of the same rhyme (the main vowel of the word), the size of the opening and the pronunciation part before and after the consonant intermediary. Wang Mechanics believes that the first and second classes have no mesolites, and the third and fourth classes have mesolites; the pronunciation parts of the first-class rhyme abdomen are backward (such as model rhyme and song rhyme), and the second and fourth grades are more pre-secondary (such as hemp rhyme and rhyme). Some of the third-class rhymes are first-class rhymes with mesolites (e.g., the first and third rhymes of the Eastern rhyme), and some are the larger pre-vowels in the opening than the fourth-class rhymes (for example, Wang Li thinks that the third-class fairy rhyme and the fourth-class rhyme both have a preposition, but the immortal rhyme belly is the first rhyme belly, and the former is larger than the latter).