I-mutation

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I-mutation (also known as umlaut, front mutation, i-umlaut, i/j-mutation or i/j-umlaut) is a type of sound change in which a back vowel is fronted or a front vowel is raised if the following syllable contains /i/, /iː/ or /j/ (a voiced palatal approximant, sometimes called yod, the sound of English ⟨y⟩ in yes). It is a category of regressive metaphony, or vowel harmony.

The term is usually used by scholars of the Germanic languages: it is particularly important in the history of the Germanic languages because inflectional suffixes with an /i/ or /j/ led to many vowel alternations that are still important in the morphology of the languages.