Today we'll discuss pairs of pants. I was all set to do some fascinating research on the etymology of the phrase and share it with you, when I discovered that The Word Detective had explained it so well that I couldn't improve upon this post. Michael Quinion had some more tidbits to add about pants and breeches at World Wide Words, however.
Using such phrases, however, remains a mystery, or at least a common source of confusion. “How,” the question goes, “can a pair of something be singular?” Certainly, when you have a pair or group of something you have more than one.
You need to remember that, when writing about a pair or pants or glasses, the subject is not “pants” or “glasses”, it's “pair”. The phrase “of glasses” tells readers what kind of pair you mean. It modifies the subject. If you remove the word pair, however, you change your verb to a plural. “My glasses are on the counter.” “My pants are in the laundry basket.”
Readers assume that, in such sentences, you mean “a pair” of them, but you could be referring to three or five, for all they know. Assuming, that is, that you regularly wear a monocle or act in accurate historical costume dramas set four hundred years ago.
Imagine the tagline above reading, "Nice pair." Now imagine a woman saying that to a stranger, as in the old commercials. Word choice makes your meaning clear.