Underwear on the outside is one of the very oldest superhero conventions, but many people still don't know why it exists in the first place.
For decades, heroes like Batman and Superman were known for, among other distinctive features, one particular costume characteristic: What looked like a pair of color-contrasting underwear on top of their costumes. We all got used to seeing red underwear over Superman's largely blue uniform, or black underwear over Batman's gray uniform, and though things like DC Comics' New 52 reboot have since removed some of this tradition, it's still something many of us associate with the image of the superhero, even if we don't know why it was there in the first place.
Well, if you're still wondering why the whole "underwear on the outside" thing caught on, or you need a quick explainer to pass on to a friend who won't stop wondering why you liked Superman's old look better, the folks at Today I Found Out have you covered. Their latest YouTube video is devoted to the underwear-on-the-outside convention, and according to them, the very first thing you need to know is this: It's not really underwear.
You see, the earliest superheroes often had looks modeled on, among other things, the circus performers and wrestlers of the first half of the 20th century, who often wore trunks over a set of tights. Since superheroes are often associated with feats of athleticism (leaping tall buildings in a single bound, that sort of thing), it was only natural that some creators would think to emulate the great athletes of their time, who often sported the "underwear on the outside" look.
For an explanation of that, plus more on the earliest superheroes, check out Today I Found Out's video below.