Advanced Tongue Root (definition)
Advanced tongue root +[ATR] is a feature used to describe sounds produced with the tongue root pulled forward. These sounds are also said to be tensed because there is a tension in the tongue and lips involved with pulling the tongue root forward. This action creates a larger pharyngeal cavity that often lowers the larynx and results in a breathy quality. Both vowels and consonants can be produced with a +[ATR] feature. In English tense vowels include /i, e, o, u/. In Scottish Gaelic, tenseness is a feature commonly found in sonorants like /L, L'/, /N, N'/, and /R, R'/ where the diacritic indicates slenderization (palatalization).
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Ladefoged, Peter (2005) A Course in Phonetics. Thomspson Wadworth Publishers.