diphthong n : a vowel sound that starts near the articulatory position for one vowel and moves toward the position for another
a complex vowel sound
joined vowels pronounced as one
In phonetics, a diphthong (also gliding vowel) (from Greek δίφθογγος, "diphthongos", literally "with two sounds" or "with two tones") is a contour vowel—that is, a unitary vowel that changes quality during its pronunciation, or "glides", with a smooth movement of the tongue from one articulation to another, as in the English words eye, boy, and cow. This contrasts with "pure" vowels, or monophthongs, where the tongue is held still, as in the English word papa.
Diphthongs often form when separate vowels are run together in rapid speech. However, there are also unitary diphthongs, as in the English examples above, which are heard by listeners as single vowel sounds (phonemes).
In the International Phonetic Alphabet, pure vowels are transcribed with one letter, as in English "sum" [sʌm]. Diphthongs are transcribed with two letters, as in English "eye" [aɪ̯] or "same" [seɪ̯m]. The two vowel symbols are chosen to represent the beginning and ending positions of the tongue, though this can be only approximate. The diacritic }} is placed under the less prominent component to show that it is part of a diphthong rather than a separate vowel, though it is sometimes left off in languages such as English, where there is not likely to be any confusion. (That is, in precise transcription,
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