Question

what is called diphthong

Answer 1

Hey there,

Diphthong, in phonetics, a gliding vowel in the articulation of which there is a continuous transition from one position to another.

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Answer 2

Hey!!
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A vowel sound with a continually changing quality may be called diphthong. A diphthong is a vowel sound consisting of an intentional glide, the organs of speech starting in the position of one vowel and immediately moving in the direction of another vowel. A diphthong, moreover, consists of a single syllable, that the vowel glide is performed in a single impulse of breath. Thus, a diphthong always occupies one syllable. If the two adjacent vowels form the nuclei of two successive syllables, they are not diphthongs. For example, boy, lay and try are diphthongs but not laying, trying, etc. because laying and trying consist of two different vowels. 

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