One of the mysteries of the English language finally explained.
1"on (also upon) all fours": (of a person) on hands and knees; (of an animal) on all four legs rather than just the hind ones. Hence "from (to,etc.) all fours": from (to, etc.) such a position.
2"to run on (also upon) all fours": to proceed smoothly; to function effectively; to succeed compatibly with.
3Chiefly with with. "to be (also stand, etc.) on all fours": to be on an equal footing; to exhibit an exact analogy or comparison; to be compatible.
4Cards. A game involving the taking of tricks, 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. Also called high-low-Jack. Compare "pitch""seven-up". Now chiefly Caribbean (in Trinidadian and Tobagan use).
Late Middle English; earliest use found in William of Palerne. In some forms from all + four, with reference to the four legs of an animal. In some forms probably showing an alteration of the some forms after fours, plural of four (perhaps compare also the adverbial ending -s); however, senses 3 and 4 may show an independent formation from all + fours, plural of four, and use of this in sense 3 may have influenced the use of the some forms in senses 1 and 2.
all fours/ˌɔːl ˈfɔːz/
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