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In phonetics, a vowel is a sound in spoken language that is characterized by an open configuration of the vocal tract where there is no build-up of air pressure above the glottis. This contrasts with consonants, which are characterized by a constriction or closure at one or more points along the vocal tract. The additional requirement is that vowels function as syllabic units: it is this criterion that distinguishes vowels from semivowels (and approximants, which in some languages may be slightly more constricted).