dipthong

Diphthong is a vowel sound that begins with one vowel quality and glides to another within a single syllable. During articulation, the vocal tract moves continuously from the position of the first vowel to that of the second, producing a contour rather than a steady state of tongue position. In many languages, diphthongs function as a distinct phoneme that contrasts with monophthongs.

In phonology, a diphthong may be analyzed as a single vowel with a glide or as a sequence of two vowels sharing one syllable nucleus. The initial portion is called the onglide and the final portion the offglide. The direction of the glide (rising or falling) depends on the relative height of the two vowels. Diphthongs are especially common in English and German, though the inventory and realization vary across languages. Some languages have very few true diphthongs and treat vowel sequences as separate vowels rather than a single unit.

Classification can be described in terms of glide direction and articulation. Rising (or closing) diphthongs move from a lower or more open vowel toward a higher one, while falling diphthongs move from a higher vowel toward a lower one. Some languages also have centering diphthongs, where the offglide is a central vowel.

Examples in English include /aɪ/ as in price, /eɪ/ as in face, /aʊ/ as in now, /ɔɪ/ as in boy, and /oʊ/ as in go. Diphthongs are often spelled with digraphs such as ai, ay, oi, oy, ou, and ow, or arise from sequences of vowels that are produced as a single syllable nucleus. The status and inventory of diphthongs vary by language and historical period.