"Some days even my lucky rocketship underpants don't help. " -Calvin
Calvin has several kinds of underpants, and preferences regarding which he would rather wear; a good-luck charm pair with a cartoon rocketship design, flyless "tightey-whitey" briefs style generic underpants, and a pair licensed with cartoon characters.
Types of underpants
Most of Calvin's Underpants were white and were seen when he dressing, or on occasion was when he tied his helium balloon onto his belt but it dangles humiliatingly from his pants fully displayed, Calvin was seen stripped to his underpants hanging on a tree branch or in the snow or Calvin taking off his pants to convince his mother to take him out to dinner.
"Lucky" Rocketship underpants
These are Calvin's favorite pair. He wears them not only as such but also as a good luck charm and confidence booster, in order to get through tough school days. Sometimes, however, they don't help, as one Sunday strip depicts him wearing them, ready to face the day, only for everything to go wrong. Sometimes the charm backfires when Calvin ripped his pants on the swings exposing his rocketship underpants on the day he has to solve a math problem at school on the chalkboard.
Cartoon character underpants
These were mentioned in a single comic, where Calvin got frustrated for being unable to wear them (they were in the laundry). At the time, he was wearing spotted shorts that could possibly be his lucky rocketship pair.
Stars and Planets underpants
Hobbes says Calvin supposes one could recognize a boy of destiny by his planet and star underpants while Calvin claims that this comment is made by a someone with less intellect who is jealous, and he is seen wearing them without his pants (Shirt, socks, shoes and the underpants he is wearing).