:Advanced tongue root:

The name of a binary feature, often abbreviated to ATR. [+ATR] sounds are produced with the root of the tongue drawn forward and the pharyngeal cavity enlarged. [+ATR] corresponds to the older feature specification [+tense] and [-ATR] to [-tense]. The long vowels in English are [+ATR], while the short vowels are [-ATR].

The above diagram is a tracing based on a cineradiographic analysis of a [+ATR] vowel (solid line) and a [-ATR] vowel (dashed line). Both vowels were produced by an Igbo speaker. The diagram is from Stewart J.M. (1967) Tongue root position in Akan vowel harmony. Phonetica 16:185-204..