VOWEL BACKNESS

In phonetics, 'vowel backness' is the position of the tongue relative to the back of the mouth in a vowel sound. Backness is usually modeled by subtracting the second formant frequency value (F2) of a vowel from the first formant frequency value (F1).
The International Phonetic Alphabet identifies five different degrees of vowel backness, although no known language distinguishes all five:

★ front vowel

★ near-front vowel

★ central vowel

★ near-back vowel

★ back vowel