中古三等韵绝大多数来自上古短元音。一般说上古每韵部或每小部(韵类)除非声母条件互补,依规则只有一个短元音来的三等韵。但在声母为以母1>j及声母带垫音j两个条件下,也有部分由长元音转来的三等韵字,从而在上古鱼铎阳三部及歌、盍、之、幽诸部形成同部三等字重出现象。本文从这两个声母条件加长元音的变化,解释了这些韵部中的麻三、清、昔韵与鱼阳药韵对立,齐、咍、盍韵出现三等字,幽部“舀陶”读宵韵与尤韵对立,“寅”归真韵而“演”变仙韵等长期令人困惑问题。 Most ancient third rhyme from ancient short vowels. Generally speaking, each ancient rhyme per ministerial or small (rhyme class) unless the initial consonant conditions complement each other according to the rules of only a short vowel to the third rhyme. However, in the consonant to mother 1> j and the parental sound pad j under the two conditions, there are also some of the third rhyme word transferred from the long vowels, so in the ancient fish Duo Yang three song and song, the quiet The formation of the Ministry with the Ministry of the other three words reappear. This paper extends the vowel changes from these two initials to explain the three, clear and prosodic rhymes of these rhymes. Tao “read the night and Yun Yun confrontation, and” Yin “to the real charm and” play "change immortal rhyme and other long-term puzzling question.