What Is "I-Mutation" or Front Mutation?
- "Early in the history of English a rule called i-Mutation (or i-Umlaut) existed that turned back vowels into front vowels when an /i/ or /j/ followed in the next syllable. For example in a certain class of nouns in the ancestor of Old English, the plural was formed not by adding -s but by adding -i. Thus the plural of /gos/ 'goose' was /gosi/ 'geese.' . . . [T]he i-Mutation is an example of a rule that was once present in Old English but has since dropped out of the language, and thanks to the Great Vowel Shift even the effects of i-Mutation have been altered." (Adrian Akmajian, Richard A. Demers, Ann K. Farmer, and Robert M. Harnish, Linguistics: An Introduction to Language and Communication, 5th ed. MIT Press, 2001)