Speech sound in which air from the lungs passes through the mouth with minimal obstruction and without audible friction, like the i in fit. The word also refers to a letter representing such a sound (a, e, i, o, u, and sometimes y). In articulatory phonetics (see articulation), vowels are classified by tongue and lip position; for example, high vowels like the i in machine and the u in flute are both pronounced with the tongue arched high in the mouth, but in u the lips are also rounded. Single vowel sounds are monophthongs; two vowel sounds pronounced as one syllable, like the ou in round, are diphthongs.

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