“A” is for Ant-Man, “B” is for Beast, “C” is for Captain America, and so on and so on. Designer Mike Boon created two parallel alphabets–upper-case for heroes, lower-case for villains–highlighting the various characters of the Marvel Comics universe. T-shirts, prints, and various other ephemera are available here and here.
I’ve got most of them down, but my C-level superhero knowledge is keeping me from some of the more esoteric villains. Anyone know who “k” is? “l”? Sound off in the comments.