A diaphoneme is an abstract phonological unit that identifies a correspondence between related sounds of two or more varieties of a language or language cluster. For example, some English varieties contrast the vowel of late (/eː/) with that of wait or eight (/ɛɪ/). Other English varieties contrast the vowel of late or wait (/eː/) with that of eight (/ɛɪ/). This non-overlapping pair of phonemes from two different varieties can be reconciled by positing three different diaphonemes: A first diaphoneme for words like late (⫽e⫽), a second diaphoneme for words like wait (⫽ei⫽), and a third diaphoneme for words like eight (⫽ex⫽).
Diaphonology studies the realization of diaphones across dialects, and is important to evaluate if an orthography is adequate for more than one dialect of a language. In historical linguistics, it is concerned with the reflexes of an ancestral phoneme as a language splits into dialects, such as the modern realizations of Old English /oː/.
