Vowel height

From KneeQuickie

Vowel height is the way in which vowels vary according to how much the tongue restricts the airflow in the mouth. Vowels with the tongue in a low position are termed "low" or "open" (because the mouth is as open as possible; this is why dentists ask their patients to say /ɑː/); vowels with the tongue in a high position are termed "high" or "close".

All natlangs distinguish at least two degrees of vowel height; most distinguish at least three. If there are three degrees of vowel height, the middle one is simply called "mid height"; if there are four, the middle two are called "mid-open" and "mid-close". The IPA works on a four-height basis (front vowels /i e ɛ a/, back vowels /u o ɔ ɑ/), but its symbols can be adapted to three-height languages by taking /e/ and /o/ as the mid vowels.

(Note: because of its tense/lax distinction, English in fact has vowels at six (in RP) or seven (in my idiolect) heights, but these are not phonemic, as the vowels are also distinguished by other factors than height.)

All languages have at least one low vowel, although most have more high vowels than low. This is simply because there is more room for the tongue to move with high vowels, and therefore more room for variation along the front/back dimension. Nearly all languages distinguish more degrees of height than degrees of frontness. (See also vowel system.)