What is "turning pair"

Pair \Pair\ (p[^a]r), n. [F. paire, LL. paria, L. paria, pl. of par pair, fr. par, adj., equal. Cf. Apparel, Par equality, Peer an equal.]

1. A number of things resembling one another, or belonging together; a set; as, a pair or flight of stairs. ``A pair of beads.''
   --Chaucer.
   --Beau. & Fl. ``Four pair of stairs.''
   --Macaulay.

   Note: [Now mostly or quite disused.]

   Two crowns in my pocket, two pair of cards.
   --Beau. & Fl.

2. Two things of a kind, similar in form, suited to each other, and intended to be used together; as, a pair of gloves or stockings; a pair of shoes.

3. Two of a sort; a span; a yoke; a couple; a brace; as, a pair of horses; a pair of oxen.

4. A married couple; a man and wife. ``A happy pair.''
   --Dryden. ``The hapless pair.''
   --Milton.

5. A single thing, composed of two pieces fitted to each other and used together; as, a pair of scissors; a pair of pants; a pair of tongs; a pair of bellows.

6. Two members of opposite parties or opinion, as in a parliamentary body, who mutually agree not to vote on a given question (in order, for example, to allow the members to be absent during the vote without affecting the outcome of the vote), or on issues of a party nature during a specified time; as, there were two pairs on the final vote. [Parliamentary Cant]

   Note: A member who is thus paired with one who would have voted oppositely is said to be paired for or paired against a measure, depending on the member's position.

7. (Kinematics) In a mechanism, two elements, or bodies, which are so applied to each other as to mutually constrain relative motion.

   Note: Pairs are named in accordance with the kind of motion they permit; thus, a journal and its bearing form a turning pair, a cylinder and its piston a sliding pair, a screw and its nut a twisting pair, etc. Any pair in which the constraining contact is along lines or at points only (as a cam and roller acting together), is designated a higher pair; any pair having constraining surfaces which fit each other (as a cylindrical pin and eye, a screw and its nut, etc.), is called a lower pair.

   Pair royal (pl. Pairs Royal) three things of a sort; -- used especially of playing cards in some games, as cribbage; as three kings, three ``eight spots'' etc. Four of a kind are called a double pair royal. ``Something in his face gave me as much pleasure as a pair royal of naturals in my own hand.''
   --Goldsmith. ``That great pair royal of adamantine sisters [the Fates].''
   --Quarles.

   Syn: Pair, Flight, Set.

   Usage: Originally, pair was not confined to two things, but was applied to any number of equal things (pares), that go together. Ben Jonson speaks of a pair (set) of chessmen; also, he and Lord Bacon speak of a pair (pack) of cards. A ``pair of stairs'' is still in popular use, as well as the later expression, ``flight of stairs.''