Diphthong

A diphthong (/ˈdɪfθɔːŋ, ˈdɪp-, -θɒŋ/ DIF-thawng, DIP-, -⁠thong;[1] from Ancient Greek δίφθογγος (díphthongos) 'two sounds', from δίς (dís) 'twice', and φθόγγος (phthóngos) 'sound'), also known as a gliding vowel, is a combination of two adjacent vowel sounds within the same syllable.[2] Technically, a diphthong is a vowel with two different targets: that is, the tongue (and/or other parts of the speech apparatus) moves during the pronunciation of the vowel. In most varieties of English, the phrase "no highway cowboy" (/noʊ ˈhaɪweɪ ˈkaʊbɔɪ/) 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. For instance, in English, the word ah is spoken as a monophthong (/ɑː/), while the word ow is spoken as a diphthong in most varieties (/aʊ/). Where two adjacent vowel sounds occur in different syllables (e.g. in the English word re-elect) the result is described as hiatus, not as a diphthong. (The English word hiatus (/ˌhaɪˈeɪtəs/) is itself an example of both hiatus and diphthongs.)

Diphthongs often form when separate vowels are run together in rapid speech during a conversation. However, there are also unitary diphthongs, as in the English examples above, which are heard by listeners as single-vowel sounds (phonemes).[3]