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/, /ī/ 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: i-mutation is particularly important in the history of the Germanic languages because inflectional suffixes containing an /i/ or /j/ led to many vowel alternations that are important in the morphology of these languages today: see Germanic umlaut.

Usage

The terms i-mutation and i-umlaut are usually applied to processes in the early Germanic languages. The phenomenon exists in many other languages, but is often referred to by different names. For example, i-mutation explains the alternations between Portuguese fiz < */fetsi/ "I did" vs. fez < */fetse/ "he did". It is still productive in some Romance languages, including Central Venetian where final -i is still visible te parchigi < */parchégi/ "you park your car" vs. parchégio "I park". But i-mutation in the Romance languages is more commonly called metaphony (from Ancient Greek, meaning "process of changing sounds", of which German umlaut is a translation). Meanwhile, in the Celtic languages, it is referred to as affection.