A vanished i pulled the vowel forward — then deleted itself, leaving the grammar fossilised in the vowel.
English keeps a strange little family of pairs where a single vowel does the work a whole ending used to do: foot / feet, mouse / mice, full / fill, long / length, old / elder. There is one law behind all of them. About fifteen centuries ago, an i, ī or j standing in the next syllable reached back and dragged the stressed vowel forward in the mouth — and then, its work done, that little vowel weakened to nothing and dropped off. The ending is gone; the change it caused is all that's left. So the rule reads backwards: you see the moved vowel and infer the lost trigger that must once have stood behind it.
Each pair below is one word in origin. The form on the right carries a fronted vowel because, long ago, it had an i or j after it that the form on the left never did — a plural, a causative, an abstract noun. Touch a pair to see the Old English forms and the ending that did it.
— the same vowel that fronts foot into feet fronts every pair here. Pick one.
This is the part that makes it a law and not a list. Every vowel had a fixed front replacement, decided only by where in the mouth it started. Pick a correspondence: the vowel slides forward to its reflex, and every doublet on the page that runs that rule lights up below. (These are the vowels at the Old English stage, where the change happened — modern spellings are these vowels carried on through later changes, the Great Vowel Shift among them.)
Pick a correspondence above to drive the rule.
Here is the whole strange move in one word, foot and feet, step by step. The plural ending arrives, conditions the vowel, then weakens and vanishes — and the plural now lives entirely in the vowel. Step it.
The cause deletes itself. This is why the rule reads backwards: you see the changed vowel (ē) and infer the lost trigger (-i) that must once have stood after it.
Given the Old English vowel and the ending that follows it, what does the vowel become? You now have the map — run it.
This section needs JavaScript; the wall, the map table and the sources above all work without it.
i-mutation is conditioned, not free. The vowel changes ONLY because an i, ī or j stood in the next syllable. That is why the singular foot keeps its ō: the singular never had *-i. No trigger, no mutation — which is exactly what makes the surviving doublets readable as fossils of lost endings.
In standard Modern English the clean mutation plurals are just feet, geese, teeth, men, women, mice, lice. They are not a living pattern: by Middle English most former mutation plurals had been re-formed on the regular -s model. Book once had the OE umlaut plural bēc — now it is books, not *beech (and beech the tree, though related to book far back, is NOT that lost plural). Oak, goat, friend, borough all had mutation plurals too, now levelled to -s. The survivors are high-frequency relics that resisted it.
The visible woman → women alternation is in the FIRST vowel, and its history is tangled (OE wīfmann → wimman, with a later vowel split), not a clean i-umlaut of the kind in foot → feet. We name it rather than use it as a clean example.
The archaic plural kine (cattle) is sometimes lumped in, but it is a different formation (a double plural of cū "cow"), not an i-umlaut relic. We exclude it by name so the set stays honest.
The same Germanic change gives German Fuß/Füße, Maus/Mäuse, Mann/Männer, lang/Länge — and German still SPELLS the front vowel with the umlaut dots ä ö ü. English merged the mutated vowels into ordinary spellings, then ran them through the Great Vowel Shift (so OE ē in fēt became the modern /iː/ of feet). The dots are gone; the change is the same.
The verifier checks the dataset is internally consistent with the fronting map and that the asterisks fall only on reconstructions. It does not re-prove Old English philology, and the pre-OE trigger forms are reconstructions (marked *), standard in the reference grammars but unattested by definition.
i-mutation is usually dated to an onset around 450–500 CE, working earliest and most completely in Old English and Old Norse, and essentially complete before the earliest Old English texts (7th–8th c.). It is shared in broad outline across West and North Germanic (English, German, Dutch, the Scandinavian languages) but absent from Gothic, the one well-recorded East Germanic language — which is why it is dated separately per language rather than as one event. The exact relative chronology, and how much was a single change versus parallel developments, is debated; we keep the dating an honest range.
The only new thing here is the form: a standard, textbook sound change made into something you can drive — apply the fronting map and watch the vowel move, step the trigger as it conditions the vowel and then deletes itself, and read a wall of Modern English doublets back to the lost endings that shaped them.
Nothing here is a new claim in historical linguistics. The map, the doublets, and the reconstructed triggers are all standard material from the reference grammars. Every pair is checked by an offline verifier (research/i-mutation/verify.mjs): that each doublet's fronted vowel really is the map's image of its base vowel, that every mutated form carries a stated i/j-bearing trigger, that the asterisks fall only on reconstructions, and that the data drawn in this page is byte-identical to the source of truth.