A single vowel sound that blends two different vowel positions, like the 'oy' sound in 'boy' or the 'ou' sound in 'house'.
From Greek 'di-' (two) + 'phthongos' (sound/voice). The word literally means 'two sounds' and was used by ancient Greek grammarians to describe vowels that slide from one quality to another.
Your mouth is actually moving while you say a diphthong—try saying 'I' slowly and feel how your tongue slides from one position to another; that movement is what makes it a true diphthong!
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