The inner series of vowels (NeiZhuan 內轉) and outer series of vowels (WaiZhuan 外轉) plays an important role in differentiating the 16 She (攝) or rhyme categories, and also has a role to play in the Deng (等) or divisions of Chinese character rhyme tables.
C l o s e
H i g h
M i d
M i d
O p e n
L o w
The above table shows some of the principle vowels. The Inner Series is represented by pale highlighted areas, whilst the rest of the table shows the Outer Series and tongue positioning.
Non-contrasting and allophone pairs are as follows.