Found:

Advanced tongue root

PHONOLOGY: a feature which characterizes phonemes which are produced by pushing the tongue root forward and often the tongue body upward, so that the resonating chamber of the pharynx is enlarged. [+ATR] vowels are [i, u, e, , o], [-ATR] are [I, U, e, , a].

LIT.	Chomsky, N. and M. Halle (1968) Halle, M. & G. Clements (1983)