Umlaut of vowels, which occurred probably in the 6th century, is also called front mutation or i/j mutation. An /i/ or /j/ in an unstressed syllable caused certain stressed vowels in the preceding syllable to be fronted or raised. The vowels affected were the Old English low vowels and diphthongs. The changes resulting from umlaut may be summarized as follows.
|/a/ + nasal||>||/e/|
|/o/, /o:/||>||/e/, /e:/|
|/u/, /u:/||>||/y/, /y:/|
|/æ'/, /æ:'/||>||/ie/, /i:e/|
|/eo/, / e:o/||>||/ie/, /i:e/|
|/io/, /i:o/||>||/ie/, /i:e/|
By an earlier change in Germanic, Germanic */e/ in this environment had already raised to Germanic */i/, which became OE /i/. By the 9th century, the OE diphthongs /ie/ and /i:e/ had begun to fall together with /i/ and /i:/, respectively, and the diphthongs were lost by the 10th century.
It is important to note that in many OE words containing vowels affected by umlaut, the /i/ or /j/ in the following unstressed syllable has been lost, so that the cause of the umlaut is not apparent unless one knows the origin of the word in Germanic.
Umlaut accounts for some apparent "irregularities" in PDE. For example, some OE nouns originally had /i/ in the plural endings but not in the singular, giving rise to "irregular" plurals such as feet, teeth, geese--where the root vowel /o:/ has mutated to /e:/ in the plural and has gone on to become PDE /i/ by the Great Vowel Shift .