"Foot" is a curious word in English because it is pluralized in an unusual way; the "oo" in the word is changed to "ee". Did this once use to be a standard way of pluralizing things in English (or a language that contributed to English), which would mean that the plural of "book" was "beek" instead of "books"? Or, is "feet" just a one-off?

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    Have a look at the i-mutation
    – Em1
    Commented Aug 10, 2012 at 13:23
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    It's down to something called i-mutation that happened in Old English. Other examples are tooth-teeth, goose-geese, louse-lice, mouse-mice. Commented Aug 10, 2012 at 13:24
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    @Em1 I don't quite understand, from that link, how the mutation happened with "feet". Did the plural of "foot" used to be "foots" and that somehow became "feets" and then "feet"?
    – Jez
    Commented Aug 10, 2012 at 13:27
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    @Jez foot/footiz (or something along those lines) > foot/feetiz > foot/feet Commented Feb 24, 2018 at 20:43
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    Of course, and moose, meese {that is a joke].
    – Lambie
    Commented Aug 12, 2020 at 15:03

9 Answers 9


Whenever you find an O (or some other back vowel like A or U) in one form of an English word and an E (or some other front vowel like Æ or I) in the corresponding place in another, you have two suspects to interrogate.

  1. If the two words are not from the same language, but from two separate Indo-European languages, like Latin and Greek (e.g, ped-al from Latin and pod-iatrist from Greek, both roots meaning 'foot' — the p ~ f and d ~ t alternations are Grimm's Law in action), then what you're seeing is E–O Ablaut. Proto-Indo-European often alternated between an "E-grade" and an "O-Grade" form for morphology, and various daughter languages inherited various words. Sort of like the distribution of family furniture when the parents die. That's not what happened here, however.

  2. If the two words are from English, and not borrowed, then what you're seeing is Umlaut. This term refers to changing a back rounded vowel [o, u] to the corresponding front rounded vowel [œ, y] in anticipation of a front vowel [i, e] in the next syllable. This is a common Germanic feature, and is still productive in German, where there are special vowel symbols (ü, ö, ä, called U-umlaut, O-umlaut, and A-umlaut) that represent this phenomenon, and these sounds. Umlaut is the guilty suspect.

In English, this happened to many normal plurals because of the E in the regular plural suffix -es. That E was pronounced in Old English and Middle English, but not in Modern English; however the root vowel had been changed already and is maintained in some, but not most, of the nouns.

The original Old English (or possibly Proto-, West, or Low Germanic) of goose/geese (in Modern English [gus/gis]) was [go:s/go:ses]. There were several steps in the derivation:

  • starting words: [go:s/go:ses]
  • final s going silent ⇒ [go:s/go:se]
  • fronting the o: to œ: by umlaut ⇒ [go:s/gœ:se]
  • derounding œ to e ⇒ [go:s/ge:se]
  • final e going silent ⇒ [go:s/ge:s]
  • Great Vowel Shift raising all long vowels one notch ⇒ [gu:s/gi:s]
  • long vowels become short ⇒ [gus/gis]
  • ending words: [gus/gis]

Edit: In this medium, where writing and typography has to express speech and sounds, I use italics and boldface like this:

  • I use plain italics only for citing examples and titles. Never for emphasis.
  • I use boldface for emphasis. These are words that would be LOUD in my speech.
  • I use bold italics for technical terms, usually with capitals, and links if I have them.
  • I also use bold italics in examples to point out individual parts that get mentioned in the text.
  • Like all linguists, I use
    • [ˌskwɛɹ'bɹækɨts] (square brackets) for phonetic data,
      distinguished from
    • /'slæʃəz/ (slashes) for phonemic transcriptions.
  • 7
    A very well detailed explanation, expressed in terms that can be understood without the need of lots of prior knowledge. Thank you.
    – Paola
    Commented Aug 10, 2012 at 19:45
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    The timing doesn't matter at all, except that presumably the vowel change developed before Proto–West Germanic split into Old English, Old High German, and Old Dutch, since all these languages had it. Commented Aug 17, 2012 at 17:36
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    I may be mistaken but I assume this answer indicates the origin of gosling too?
    – Jodrell
    Commented Jan 7, 2013 at 16:03
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    Had the Anglo-saxon plural béc survived, it would probably appear in modern English as beech rather than beek.
    – Colin Fine
    Commented Mar 10, 2013 at 21:47
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    Just noticed this now: I think you're conflating two plural suffixes here. The umlauting suffix is inherited PG -iz (< PIE -es), whose final sibilant was lost in West Germanic, and the final vowel some time before ‘classical’ Old English, after it caused umlaut. The Modern English -s suffix only became -es during late Old and early Middle English—it is the Old English suffix -as (< PG -ōs) found in strong a-stem masculines (the largest group), which is why it never caused any umlaut. Those that have umlaut are the last remnants of the original -es > -iz > group. Commented Jul 30, 2016 at 10:22

Here is a quote from the Old English dictionary:

bóc [] f (béc/béc) 1. a book, a document, register, catalog; 1a. a legal document, (1) a bill of divorce; (2) a charter; (3) a title deed; (4) conveyance; 2. a book, volume, literary work, pages; main division of a work;

Let's now look at:

fót [] m (-es/fét) 1. a foot; 2. the foot, the foot of a man

So, I guess yes, we can say that at least the word from which modern book evolved had plural in the same form as the word from which modern 'foot' evolved. I guess, as early of Middle English period, "book" lost its irregular plural, since Chaucer used bookes.

It's also interesting to notice that fót actually had an alternative form fótes, but this form has not survived.

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    Consider that German has Buch / Bücher (Plural with Umlaut). But Wort has a double plural: Wörter / Worte (with different meanings), i.e. it has a plural with Umlaut and a plural without Umlaut. So probably both kinds of plurals were wide-spread in different Germanic languages.
    – Giorgio
    Commented Aug 11, 2012 at 20:47

According to the Online Etymology Dictionary this is an instance of I-Mutation

I-MUTATION (also known as "i-umlaut") is the raising and fronting of a root vowel in anticipation of "i" or "y" sound in a suffix.

Other examples of noun plurals that exhibit I-Mutation are

man → men
tooth → teeth
goose → geese
louse → lice
mouse → mice

I-mutation is caused by the very human habit of laziness: taking the shortest distance between two points. The plural of man in ancient West Germanic, the ancestor of Old English, used to be a word something like *manniz. The speakers "cheated" on the first vowel in the word to be in position for the second vowel. It's the same thing you do with doing. It doesn't change the meaning of the word to do so.

So, it comes from the old Saxon roots of English. Book, being a relatively new and uncommon noun compared to foot, probably wasn't shortened so commonly.


As a rule, morphological irregularities of Modern English are remnants of previously regular processes of Old English.

So, if English had been a more conservative language, we would have retained beech as the plural of book. Moreover, we would have had book as the past tense of bake (like took and shook from take and shake).

German, which is more morphologically conservative than English, still has vowel alternation in both these places. The plural of Buch “book” is Bücher, and the past tense of back- “bake” is buk (though a regular form, backte, is emerging, just like English baked).


This will be a short answer, because I cannot research the topic right now.

I don't know whether in Anglo-Saxon times book, or better its equivalent, might have had a plural form similar to the one foot has nowadays. What I know from reading Chaucer's "Canterbury Tales" (14th century English) is that at that time the word was spelt "bookes" in the plural, as you can read in the passage about the Oxford Cleric :

For hym was levere have at his beddes heed
Twenty bookes, clad in blak or reed,
Of Aristotle and his philosophie ...

Besides, the plural form in double 'e' is surely not a one-off as you suggest; the same is true for tooth - teeth and goose - geese (and perhaps something else which does not come to my mind right now).

Finally, we should remember that there are words which are written with two o's but the corresponding verb is written with two e's (for example, blood and to bleed)

  • 2
    That does nothing say about why the plural of book became books, while the plural of foot became feet.
    – Em1
    Commented Aug 10, 2012 at 13:43
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    @Em1. That's true, but actually the OP asked if that was the standard way of pluralizing words in English, and I reported that in Middle English the plural of book was bookes; then he also wondered whether feet was an exception, which is not the case, as FumbleFingers indicated as well.
    – Paola
    Commented Aug 10, 2012 at 14:24

The plural of foot always was feet; we inherited the word from West Germanic, which seems to have had a lot more plurals where the vowel changed than English does today. The word for "foot" in German is Fuß (rhyming with "moose"); the plural is Füße. You see the umlaut: that means the vowel changes, just like it does in English. The word for book in German is Buch; the plural is Bücher, so you see the vowel changes in that as well. Somewhere along the way, English lost the irregular plural of book, and most of the other vowel-changing plurals as well. Only a handful in very common words survived.

  • What does "irregular" mean here ... not conforming to the rules that are still active? Because in a sense, "beek" as a plural of "book" would have been the regular plural according to the rules operating at one time.
    – LarsH
    Commented Aug 10, 2012 at 18:25
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    @LarsH: I guess I was assuming that at the time English lost the irregular plural of book (based on the other answers, somewhere between Old English and Middle English), vowel-changing plurals were already becoming irregular. Commented Aug 10, 2012 at 18:28

All the answers above are excellent but I'm adding this just as a proof that the plural of "book" was "beek" in Old English.

David Crystal writes in The Cambridge Encyclopedia of the English Language p19 that the plural of book was bec at one point, but it didn't survive into modern English due to analogy with so many other regular verbs.

The relevant paragraph is:

There are a few exceptions and complications, which analysts still puzzle over, but the general effect on the language was immense, as this sound change applied to the most frequently occurring word classes, all of which had i sounds in their inflectional endings. This is why we have in Modern English such pairs as food/ feed (from the addition of an *-ian verb- forming suffix in Germanic), as well as strong/ strength and several others (from the addition of an *-iþ adjective-forming suffix). Not all the forms affected by i-mutation have survived into Modern English, though. In Old English, the plural of book was bec, but this has not come through into Modern English as beek: the forces of analogy (p. 212) have taken over, and caused a change to the regular books.


Old English bōc did indeed become bēċ by umlaut. I don't know when the -s form rose, whether it was already present in OE or not (bōcas?), but that form eventually won out. Other words which lost their umlauts include oak (OE āc/ǣċ) and nut (OE hnutu/hnyte).


In the old languages, such as Old English and Old Norse, which are the largest sources for modern english, book was derived from Bok. As foot was from fot. Multiples were boks and fots. By speaking these words in a fast and lazy manner, as people do in regular conversation, the words eventually evolved into Beeks and beek for the plural book, and feets and feet for the plural foot. During the transition of the 13th century, the english language started becoming more organized. English kings were all speaking old french, but demanded a unification of the English language for better unity of the people to be ruled. By the end of the 13th century, beek was no longer common and had been plurilized by an "s", thus we now use books. Feet remained in common language because everyone who spoke english mention foot and feet on a near daily basis. Books were not so common to the average englishman. The words that kept the oral mutation, such as foot and feet, woman and women, mouse and mice, etc, were all words used on a daily basis by the common man of the time.

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    This is complete nonsense. The plural of "foot"/"fot" was never "fots". The vowel mutation came about due to a regular process, not laziness. And kings made no decrees about the organization of the English language, if they even cared about the issue at all. The language remain splintered into dialects for quite some time.
    – siride
    Commented Aug 18, 2013 at 4:07

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