Vowels: Three-way contrast on front-back dimension
Languages typically make more contrasts on the open-close than the front-back dimension.
Thus it is quite unusual to find a three-way contrast such as the one shown here
between back, central, and front rounded vowels. English /u/ is of course often
quite central, i.e advanced from a cardinal [u] position, but in English there is
no contrast with a front rounded vowel to be maintained.
biː ("next to")
The Sounds of the World's Languages.
Ladefoged, P. & Maddieson, I. (2008): The Sounds of the World's Languages. Malden, MA: Blackwell.