Alexander MacBain,
An Etymological Dictionary of the Gaelic Language
(1911)
“ In Gaelic the vowel or vowel combination of a syllable may undergo “mutation” (German umlaut) in the course of inflection or word-building. This mutation is caused by the influence exerted backward by the vowel of the next syllable now or previously existent. There are three classes of mutation in Gaelic caused either by a following (1) e or i, (2) a or o, or (3) u. ”
