An open vowel is a vowel sound in which the tongue is positioned as far as possible from the roof of the mouth. Open vowels are sometimes also called low vowels (in U.S. terminology ) in reference to the low position of the tongue.
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Vowels beside dots are: unrounded • rounded
In the context of the phonology of any particular language, a low vowel can be any vowel that is more open than a mid vowel. That is, open-mid vowels, near-open vowels, and open vowels can all be considered low vowels.