Jeans

Jeans are trousers made of blue-dyed denim (see Wikipedia:Denim. The original and classic were done by Levi-Strauss, with copper rivets, and until the advent of designer jeans in the early 70s, owned the market. In the Leather scene, 501 Levi's, with metal fasteners in the crotch, still remain a de regueur fashion statement.

With 501s, it was the fasteners that first drew the gay scene to it. You could pull your piece out without chafing by a zipper, which meant you could wear them naked underneath.

Fashion jeans were tighter, but generally used a zipper, in that the expected market was not into gay jeans-sex (essentially oral).