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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/, /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.
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. However, 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.
I-mutation took place separately in the various Germanic languages starting around 450 or 500 AD in the North Sea area and affected all of the early languages except for Gothic. It seems to have taken effect earliest and was most complete in its implementation in Old English and Old Norse. It took place later in Old High German: by 900, its effects are consistently visible only in the spelling of Germanic */a/.