A-mutation was a vowel harmony process that took place at the late Proto-Germanic stage (perhaps around 200 AD), and caused a high vowel to become lowered when a following syllable contained a non-high vowel (not just /a/; hence the term is a misnomer). It did not occur if a /j/ in a consonant cluster nasal followed by another consonant intervened. It operated most consistently on /u/, which was usually lowered to /o/ (a sound that did not previously exist, since /o/ from Proto-Indo-European had become /a/ by an earlier change). Its most noticeable effect was on past participles, where, for example, Old English 'flogen' "flown" < */flugana/ alternated with 'flugon' "they flew" < */flugunþ/. However, it is also visible in a few lexical alternations, for example 'gold' "gold" < */gulda/ vs. 'gyldan' "to gild" < */guldjan/.

U-mutation was a change in Old Norse occurring after i-mutation where a vowel was rounded when a /u/ occurred in the following syllable.

sound laws | Germanic languages