Germanic a-mutation

A-mutation is a metaphonic process in late Proto-Germanic.
In a-mutation, a short high vowel was lowered when the following syllable contained a non-high vowel.
The term a-mutation is a misnomer, since it is not limited to a-vowels.
A-mutation is also called «a-umlaut», «a/o-umlaut», «velar umlaut» and «Brechung».
A-mutation preceded the raising of unstressed final oː to uː in the dialects ancestral to Old English and Old Norse.
Dialectal variation in a-mutation is evident in Old High German, West Germanic languages and Old Norse.
A-mutation is less evident in Old East Norse and Gothic.
Crimean Gothic may have preserved a-mutation.
A-mutation affects the vowels i and u, with i > e and u > o being the most common changes.
The diphthong eu was affected by a-mutation in the West Germanic variety that gave rise to Old English.
Effects of a single nasal consonant were also observed in Old English and other West Germanic languages.

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