With the method of dividing part of rhymes of Tangyun (唐韻), Gu Yanwu (顧炎武) established the first scientific system of Old Chinese rhyme group. However, because he did not get rid of the influence of the theory of near-rhyme (韻緩説), his ten groups are not proper rhyme groups. Jiang Yong (江永) abandoned the theory of near-rhyme and clarified the definition of rhyme group, which was a development of the study on Old Chinese phonology. However, due to the limitation of Mandarin pronunciation, Jiang's division of rhyme groups is not perfective. Duan Yucai (段玉裁) broke the boundaries of Mandarin pronunciation, which was another great development of the study on Old Chinese phonology.