Why are underwear and pants always referred to as a "pair"? Yes, there are two leg holes or pant legs, but a single shirt has two arm holes or sleeves but is never referred to as a pair of shirts.
-- Apparelly Challenged, Poway
Apparently so. "Pair" from the Latin means two like things. And pants (pantaloons) were originally two like things. You put them on one leg at a time because they actually came in two pieces. You put on one leg, tied it around your waist, put on the other leg, tied it around your waist. From the beginning, about the 16th Century, pants (and breeches, trousers, etc.) have been referred to as a pair. Shirts, on the other hand, were made from a single piece of cloth, so two sleeves didn't inspire anybody to call it a pair of shirts. Or a pair of bras.