The three rhyme categories *e（支部），*i（脂部） and *（之部）were mutually independent in the early Ancient Chinese, and this proposition has been confirmed by different kinds of materials. But the dialects in late Ancient Chinese were indeed showing a number of mixed cases. Some scholars claim that the three rhyme categories were a whole at an early stage. This skepticism neither recognizes the fact that the three rhyme categories were clearly demarcated, nor does it offer a scientific explanation for the mixed cases from the perspectives of time and space. In fact, the mixture was the result of historical development and was gradually realized through conditional sound evolution. The evolution process was not only diachronic — connecting the earlier ancient Chinese dialects with the later ones, but also synchronic — displaying the unbalanced regional development in the geographical dialect categories. The most significant regional difference is the one between the Southern and Northern geographical categories. In North China, the *e（支部） and *i（脂部） categories are interlinked, and are therefore grouped under the ″Northern Category″. The ″Northern Category″ can be divided into two stage subtypes — ″Qin Jin Category″ and ″Qilu Category″, according to the differences of sound conditions. In the southern region, the *i（脂部） and *（之部） are closely related, and are grouped under the ″Southern Category″. The essence of the rhyme transformation in dialects is sound change, which is restricted by specific phonological rules and phonetic conditions.（1）*e（支部）>i/C［+acute］ was the essence of the phonetic evolution of the ″Qin Jin Category″.（2）Chain change ->-n，-n>-/V ［-back occurred in the Qilu region, thus *e>en（耕部），and *en> e（元部）. Under this sound change pressure, *e（支部） had to rise to（高化） i. Consequently, *e>i （支部）merged with *i（脂部） when the front sound was an unlabial initial.（3) Expansion of the ″Northern Category″：″Qilu Category″ might have evolved from the ″Qin Jin Category″. Based on the ″Qilu Category″ it expanded the ″Qin Jin Category″ and merged ie（支韵）and i（脂韵） in late ancient Northern China. （4) *?（之部）>i/C［+acute］〖CD#1〗 is the essence of the phonetic evolution of the ″Southern Category″.（5) Regional overlap of the Southern and Northern categories: as Qilu and Bashu had been impacted by both the Northern Classic Dialect and Southern Chu Dialect, the two regions showed an overlap of the Southern and Northern geographical categories.