Why is the word "pants" plural?  Some people say it's because pants contain two pant legs, hence a "pair of pants."  But my shirt contains two arms; does that make it a pair of shirts?  No. 

This is an example of a tricky plural.  Other examples include a pair of scissors, a set of eyeglasses, and underwear. 

Also related:  A "TV set" is so called because apparently the components used to come in a set, and the user had to put everything together. #language