||The definition of Inner turning 內轉 and Outer turning 外轉 in early rhyme tables and the directions of Inner and Outer in menfa failed to provide with us the explicit phonetic property of these terms. On the contrary, they are but ambiguous or even contradictory phonology descriptions. Opinions of modern scholars also differ from each other. These opinions can be grouped into two major schools, namely, the “phoneticists” and the “divisionists”. The current study considers the issue of Inner/Outer turning on the basis of a thorough trace back of the origin and evolution of them. The prototype of Inner/Outer turning is originated from the [+low]/[-low] contrast in terms of distinctive features of two types of vowel phonemes in OC. In EMC, as the result of series of sound changes, the former [-low] vowel phonemes turned out to have Division I and/or Division III syllables only, whereas the former [+low] phonemes may contain Division II and/or Division IV syllables too, thus presenting a picture very similar with that of the Inner/Outer turning structure of the rhyme tables. The definition of Inner/Outer turning appears as late as the Song times. Yet the prototype of the Inner/Outer contrast is a phonetic reality of OC. It is in EMC that the Inner/Outer contrast brought into full play its role. With the elimination of the long/short contrast and the –r- segment in EMC and the continuous inter-shifting of the members between Inner and Outer, the concept is already an out-dated and ambiguous mixture by the time it is created. That is why we got all those inconsistent and self-contradictory interpretations over the history. However, interpretations of modern scholars, either the “phoneticists” or the “divisionists”, have their own particular emphasis and blind points. That is what we shall be aware about when using these concepts. Keywords: Inner turning, Outer turning, origin, [+low] / [-low] vowel contrast, Early middle Chinese.