Germanic languages like Old English had an “umlaut” rule that changed a back vowel to a front vowel if the next syllable contained a high front vowel sound. For example, in foti, the plural of “foot,” the back o was altered by the rule to a front e, harmonizing with the front i. Subsequently the i at the end ceased being pronounced, and because the phonological rule no longer had anything to trigger it, speakers reinterpreted the o–e shift as a morphological relationship signaling the plural—resulting in our foot–feet, mouse–mice, goose–geese, tooth–teeth, and louse–lice.
