monophthongization

Monophthongization is a phonological process in which a diphthong or vowel sequence is realized as a single, stable vowel. Diphthongs are vowels that begin with one articulation and glide toward another; monophthongization retains only a single vowel quality, eliminating the glide. The result may be phonemic, creating a distinct monophthong that contrasts with other vowels, or allophonic, with the outcome depending on context.

The change can be unconditioned, occurring broadly across a language, or conditioned, restricted to particular environments such as before certain consonants, within closed syllables, or in stressed versus unstressed positions. Analogy, language contact, and sociolinguistic factors can also drive or reinforce monophthongization, producing regional or social varieties.

In phonetic terms, a diphthong like [aɪ] may be realized as a long vowel such as [aː], or as another single vowel such as [eː] or [oː], depending on the language. The process reduces vowel inventory and often interacts with vowel length, stress, and prosody. In some languages, the phonetic result is a centralized or mid vowel rather than a long vowel.

Monophthongization has occurred in many languages throughout history and is documented across diverse language families. It is a common explanation for shifts that stabilize or simplify vowel systems in diachronic phonology and in contemporary dialectology, where formerly diphthongized vowels are realized as single vowels. See also vowel, diphthong, vowel shift.