Diphthongos

Diphthongos are a type of vowel sound in which the articulation moves within the same syllable from one vowel quality to another. The term comes from the Greek diphthongos, meaning "two sounds." In many languages, a diphthong is treated as a single phoneme, even though its acoustic quality changes during production.

During articulation, the speaker glides from a starting vowel to a second vowel, producing a vowel sound with two distinct qualities in one syllable. This makes diphthongs different from hiatus, where two vowels occur in adjacent syllables, and from pure vowels that maintain a single quality. In English, common diphthongs include /eɪ/ (as in face), /aɪ/ (bite), /ɔɪ/ (boy), /aʊ/ (now), and /oʊ/ (go). Diphthongs can be described as rising or falling, depending on how the second vowel relates to the first.

Cross-linguistic variation is common: some languages have many diphthongs, others have few or none. In some Romance languages, sequences of vowels within a single syllable behave like diphthongs with a glide, while other languages use two adjacent vowels in hiatus. Diphthongs are also distinguished from triphthongs, which involve a glide through three vowel qualities within a single syllable.