pair

1250–1300; Middle English paire < Old French < Latin pāria, plural (taken as feminine singular) of pār a pair. See par¹

1. Pair, brace, couple, span, yoke are terms for groups of two. Pair is used of two things naturally or habitually associated in use, or necessary to each other to make a complete set: a pair of dice. It is used also of one thing composed of two similar and complementary parts: a pair of trousers. Brace is a hunter's term, used of a pair of dogs, ducks, etc., or a pair of pistols or slugs: a brace of partridges. In couple the idea of combination or interdependence has become greatly weakened; it may be used loosely for two of anything ( a couple of apples ), and even for more than two: I have to see a couple of people. Span is used of a matched pair of horses harnessed together side by side. Yoke applies to the two animals hitched together under a yoke for drawing and pulling: a yoke of oxen.

When used without a modifier, pairs is the only possible plural: Pairs of skaters glided over the ice. When modified by a number, pairs is the more common form, especially referring to persons: Six pairs of masked dancers led the procession. The unmarked plural pair is used mainly in reference to inanimate objects or nonhumans: He has three pair (or pairs ) of loafers. Two pair (or pairs ) of barn owls have nested on our property.
Pair signifying two individuals can take either a singular or plural verb, but it is usually followed by a plural verb and referred to by a plural pronoun: The guilty pair have not been seen since their escape.
In the sense “a set or combination of more than two objects forming a collective whole,” pair occurs chiefly in fixed phrases: a pair of beads; a pair of stairs. This use is now somewhat old-fashioned. See also collective noun, couple.