In phonetics, a diphthong, or /ˈdɪp.θɒŋ/, (also gliding vowel) (from Greek, diphthongos, literally "two sounds" or "two tones")'s a contour vowel—that's, 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's held still, as in the English word papa. Diphthongs… (More on Diphthong)