A diphthong, also known as a gliding vowel, is a combination of two adjacent vowel sounds within the same syllable. Technically, a diphthong is a vowel with two different targets: that is, the tongue moves during the pronunciation of the vowel. In most varieties of English, the phrase "no highway cowboys" has five distinct diphthongs, one in every syllable. Diphthongs contrast with monophthongs, where the tongue or other speech organs do not move and the syllable contains only a single vowel sound.

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