Vowel harmony is a type of assimilatory phonological process involving vowels separated by consonants––i. e., not adjacent to each other––that occurs in some languages. In languages with vowel harmony, there are constraints on which vowels may be found in adjacent or succeeding  syllables. For example, a vowel at the beginning of a word can trigger assimilation in a vowel at the end of a word. The Uralic group (like Finnish and Hungarian) and Turkic (like Turkish and Tatar) are prominent instances of language families with vowel harmony. Thus in Hungarian, város ‘city’ has the dative form városnak, whereas öröm ‘joy’ has örömnek: the dative suffix has two different forms -nak/-nek. The -nak form appears after the root with back vowels (a and o are both back vowels), whereas -nek appears after the root with front vowels (ö and e are front vowels).

English does not have vowel harmony as a regular phenomenon, but a trace of this process may account for the strange case of the common mispronunciation in both British and American English of the verb lambaste as [lambást], which is a compound consisting of the verbs lam and baste, both of which mean ‘to beat soundly, thrash, cudgel’. Since the pronunciation of the second unit baste is invariably [béyst], the pronunciation of the compound ought not to vary from [lambéyst], but does anyway. Only vowel harmony, where the vowel of lam influences that of baste, suggests itself as an explanation for this deviation.