To walk or crawl "on all fours" means to get about on hands and knees like a four-legged animal, or the process of locomotion by such an animal itself.

The word four can be used as a determiner to describe a quantity of items, in which case the noun is plural: four bricks, four beers, four colors. But somehow, in the expression "on all fours" the plurality has shifted from the noun (presumably legs in this case) to the determiner itself. I can think of no other case where this has happened.

From Etymonline's entry on four:

To be on all fours is from 1719; earlier on all four (14c.).

So somehow it got from a shortening of (again presumably) "on all four legs" to "on all four"; then at some point during the next several hundred years, mirabile dictu, somehow the s migrated from legs to fours.

I'm looking for where and how this could have happened. Please do not respond to tell me what the phrase means or anything like that. I only want to know how the s got transferred from noun to number, or examples of other words that have undergone a similar progression, should any exist.

  • 1
    I suspect a classic case of contamination: on all four existed, and on four legs existed; then at some point they were conflated and the result somehow gained traction, on all fours. Commented Sep 15, 2015 at 1:16
  • all fours has some hints of being nautical-- moored unswinging-- which gave it a head start in the eighteenth century. Some sources say literally two lines fore and two lines aft, which is suppose would have been illustrative of being on hands and knees.
    – stevesliva
    Commented Sep 15, 2015 at 5:05
  • 2
    I don't think you need to posit a shortening from 'four legs' to 'fours'. It can be simply a metonymy of 'four' for each limb and there are a plural number of them. That's how it feels to me.
    – Mitch
    Commented Sep 15, 2015 at 12:26
  • 2
    Perhaps "blues" is comparable to "fours", in terms of dropping the noun and appending the plural endling to the modifier: the etymology section of Wikipedia entry for "blues" (en.wikipedia.org/wiki/Blues#Etymology) suggests it derives from "blue devils".
    – anemone
    Commented Sep 15, 2015 at 20:54
  • 1
    @Mitch I would actually say that in threes is quite fundamentally different: a three in that use is (apart from awkward) a group of three things, and threes are several groups of three. That makes it parallel(ish) to on all four, but it also means that on all fours ought to refer to all the groups of four in the world or something like that—which might work if you're a millipede, but hardly for most humans. So the pluralising here is indeed very odd: the noun is pluralised, but the meaning is not. Commented Sep 15, 2015 at 20:57

3 Answers 3


In the OED Third Edition, September 2012, the (updated) etymology given under the headword all fours, n. suggests three routes for the -s attachment.

  1. With reference to the four legs of an animal, the alteration of 'all four [legs]', adj. + adj., to 'all fours' might be explained by the omitted but understood plural noun, 'legs'. The earliest use evidenced for 'all fours' in this sense is 1678.

This is probable.

  1. The -s suffix forming adverbs may also have played a role:

... Hence there arose in early Middle English mixed forms such as aȝeines, amiddes; and the frequent coexistence of the two forms of the same adv., one with and the other without s, led to the addition of s to many advs. as a sign of their function. In some instances the extended form prevailed, as in eftsoons; in others it survived only in dialects, as in oftens, gaylies (Scottish).

This is perhaps probable.

  1. The earliest use of 'all-fours', in 1674, refers to the name of a card game:

... in which four game points are available per round: for being dealt the highest trump, being dealt the lowest trump, taking the Jack of trumps, and winning the most tricks.

About this sense, the OED suggests that its use "may have influenced the use of the" -s forms in both the sense 'on or upon all fours' with reference to the four legs of animals and the sense 'to run on or upon all fours'. The OED also suggests that this sense "may show an independent formation" from the adj. 'all' and the plural of the noun 'four'.

This is possible.

After looking at the evidence and reading the analysis presented in the OED, I favor the explanation that all of the suggested influences played a role in the adoption and retention of the -s form. Of those influences, I am biased toward the last, the use and historical prevalence of the name of the card game 'All-Fours', as the most dominant. My bias arises mostly from that being the earliest evidenced use of the -s form.

  • Well-researched, well-reasoned, well-argued.
    – Robusto
    Commented Sep 16, 2015 at 10:53
  • 2
    I just found two apparent earlier mentions of "on all fours", but both turned out to be "(up)on all foure", after considerable investigation. One in 1611, the other in 1618. I was very excited for half an hour. Commented Sep 16, 2015 at 11:13
  • In case anyone's interested, here are the links: hdl.handle.net/2027/nc01.ark:/13960/… and books.google.co.uk/… Commented Sep 16, 2015 at 12:45
  • @PhilMJones, red herrings on all fours, an evolutionary oddity!
    – JEL
    Commented Sep 16, 2015 at 20:02

The World Wide Word suggests that the "s" was introduced in the 19th century suggesting the four legs or extremities:

  • The image behind it is that of a dog or similar animal. If it has use of all four legs, it runs smoothly and evenly, as opposed to the way it would limp if one of its legs were damaged. The expression was originally on all four, known from the sixteenth century in phrases we’re still familiar with, such as to crawl upon all four (the final “-s” was added in the nineteenth century; earlier, a word such as “legs” or “extremities” was understood).
  • In the eighteenth century, people started to use to run on all four as a figurative expression to describe some proposition or circumstance that was fair or equitable, well-founded, sturdily able to stand by itself. To be on all four or to stand on all four meant to be on a level with another, to present an exact analogy or comparison with something else (presumably the image is of two animals standing together, both on all four legs, hence in closely similar situations).

  • It’s hardly common now outside the legal profession, and I suspect from what subscribers have told me that it doesn’t turn up that often these days even in that field.

This is confirmed by The OED which notes the 19th-century s-addition,

  • [formerly all four, sc. extremities. The -s was added prob. during the 19th century; not in Johnson 1808.]

  • invokes a metaphor of the form "not limping = fair or even, not lame", and gives an earlier citation, from a British legal context, which also involves running, and is applied to a comparison:

    • fig. to run on all fours, i. e. fairly, evenly, not to limp like a lame dog. to be, or stand, on all fours: to be even or on a level, to present an exact analogy or comparison (with).

    • 1877 Daily Tel. 15 Mar., It must stand on all fours with that stipulation._

    • [1883 Daily News 8 Feb. 3/7 The decision I have quoted is on all fours with this case.


  • OED 3rd Ed. gives quotes showing the -s from the late 1600's and elaborates the etymology.
    – JEL
    Commented Sep 15, 2015 at 8:04
  • +1 for the work, but can you shed any light on the actual mechanism involved in the transference?
    – Robusto
    Commented Sep 15, 2015 at 14:21
  • @Robusto - On all fours: On one's hands and knees, as in 'Seven of us were on all fours, looking for the lost earring in the sand'. In this idiom fours refers to the four limbs. [1300s] . (The American Heritage® Dictionary of Idioms ). The extract by Mark Liberman I linked to my answer is the most comprehensive analysis I could find on this issue. The only comment on the move from "four" to "fours" is actually by Michael Quinion on WWW, who suggests a possible change from "on all four legs/extremities" to the shorter "on all fours". It appears to be a plausible explanation.
    – user66974
    Commented Sep 15, 2015 at 19:00

We say "modal verbs" and often speak in shortened form of modals. Though "modal" is an adjective we add the plural-s of "verbs" to "modal" and create a new noun in plural.

I think that the same process took place when "moving on all four limbs/extremities" was shortened to "moving on all fours".

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