in "a"-mutation, a short high vowel ( or ) was lowered when the following syllable contained a non-high vowel (, or ). thus, since the change was produced by other vowels besides */a/, the term "a"-mutation is something of a misnomer. it has also been called ""a"-umlaut", ""a"/"o"-umlaut", "velar umlaut" and, formerly, "brechung". (this last was grimm's term, but nowadays german "brechung", and its english equivalents "breaking" and "fracture", are generally restricted in use to other unrelated sound-changes which later affected individual germanic languages.)