Why do we say a "pair of pants" when there's only one of them?
Why do we say "a pair of pants" when there is only one article of clothing involved? I have been told it's because there are two legs, but then why isn't it a pair of shirts? Shirts have two sleeves. I'm so confused. Can you help?
Fret not, my little anchovy. Ann Landers might puppy out and tell you to get professional counseling, but here at the Straight Dope, we deliver.
Now for the facts.
First of all, let's note there is a class of objects that are thought to consist of two independent but connected parts, usually identical or at least similar to each other. In addition to pants and trousers, there are eyeglasses, scissors, tweezers, shears, pliers, and so on.
The terms for these objects are always plural in form, and they are usually referred to as "a pair of ...." This usage goes back to at least 1297 AD, when we have the expression "a peire of hosen."
The implication is that the two parts are separable in some sense, and in fact a pair of hose can often mean two separate pieces. (True, you can't separate tweezers, but I never claimed the English language was rational.)
In contrast to trousers, a shirt is thought of mainly as a covering for the torso, and may or may not have sleeves. Hence no pair.
The "pair of ..." designation is somewhat arbitrarily applied. At one time it was common to speak of a pair of compasses (for drawing), a pair of nutcrackers, or a pair of bellows. But I would venture to say that in the U.S., at least, these expressions are dying out.
On the other hand, we do speak of a pair of panties, even though panties aren't really a pair of anything, having (usually) no legs. But clearly this is merely an extension of the expression, "pair of pants."
Further confusing matters is "a dozen pairs of rosaries," even though there are 50-some beads. This harks back to an old use of the word "pair" to mean "a set of more than two like or equal things making a whole."
A related usage, supposedly common in the theater business for many years, is "a pair [flight] of stairs." Occasionally theatrical types will say of a pair that it is "nice," and one assumes the rest of the superstructure ain't bad either. But that's a discussion for another day.