Alphabet Blogging: brought to you by the letter P

P is for Pajama Pants.
I have made these 4 pairs thus far.
Here are my handsome/silly models:

By the way, why are they called pants? I mean why the plural, "pair of pants"?
It is only one article of clothing. Do you suppose pants used to come in two separate parts?
I have 2 arms, why not call a sweater a "pair of sweaters"?