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A mid vowel (or a true-mid vowel) is any in a class of vowel sounds used in some spoken languages. The defining characteristic of a mid vowel is that the tongue is positioned midway between an open vowel and a close vowel.
Other names for a mid vowel are lowered close-mid vowel and raised open-mid vowel, though the former phrase may also be used to describe a vowel that is as low as open-mid; likewise, the latter phrase may also be used to describe a vowel that is as high as close-mid.
The IPA divides the vowel space into thirds, with the close-mid vowels such as [e] or [o] and the open-mid vowels such as [?] or [?] equidistant in formant space between open [a] or [?] and close [i] or [u]. Thus a true mid front unrounded vowel can be transcribed as either a lowered ⟨e?⟩ (with a lowering diacritic) or as a raised ⟨⟩ (with a raising diacritic). Typical truly mid vowels are thus:
Few languages contrast all three heights of mid vowel, because it is rare for a language to distinguish more than four heights of true front or back vowels.
The Kensiu language spoken in Malaysia and Thailand is highly unusual in that it phonemically contrasts true-mid vowels with close-mid and open-mid vowels without differences in other parameters such as backness or roundedness.