Jeans

Jeans are trousers made of demim, usually dyed blue (see Wikipedia:Denim). The original and classic jeans were made by Levi-Strauss, with copper rivets, and until the advent of designer jeans in the early 70s, Levi-Strauss dominated 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 them. You could pull your piece out without danger of chafing by a zipper, which meant you could wear them with no underwear.

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