diphthongize

Diphthongize refers to the process by which a vowel becomes a diphthong, a vowel sound that involves a smooth movement from one quality to another within the same syllable. The term can describe a historical change in a language or a contemporary phonetic realization.

Phonetically, a diphthong consists of an onglide (the starting vowel) and an offglide (the ending vowel). Diphthongs can be phonemic, serving as distinct vowel phonemes that distinguish words, or allophonic, realized variably across dialects or contexts.

Diphthongization is common in many language families and often arises due to contact, phonotactic constraints, prosodic factors, or articulatory simplification.

Examples include English diphthongs such as /aɪ/ in eye, /eɪ/ in face, /aʊ/ in mouth, /ɔɪ/ in boy, and /oʊ/ in go. In some languages, historical vowel changes have turned monophthongs into diphthongs, while others preserve non-gliding vowels.