In historical linguistics, metaphony is a general term for a class of sound shift in which one vowel in a word is influenced by another in a process of assimilation.

We can distinguish progressive metaphony, in which a vowel early in the word influences a subsequent vowel, from regressive metaphony, in which a vowel towards the end of the word influences a preceding vowel. (Progressive metaphony is sometimes called "left-to-right" metaphony, and regressive metaphony may be called "right-to-left" metaphony. These terms run into problems, however, when describing languages such as the Semitic family, which use scripts running in the opposite direction.)

Progressive metaphony is also called vowel harmony, and is discussed fully in the article under that heading. However, some linguists use the term "vowel harmony" for regressive metaphony too.

Examples of regressive metaphony are i-mutation and a-mutation. (On i-mutation in Germanic languages see also umlaut.)

Historical linguistics

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