Nylons are composed of polymers containing repeated amide groups called polyamides that have a high molecular weight. When produced as fibers, nylons are elastic and strong. If these fibers are stretched into fine filaments, they become lustrous and can be spun into a material that is similar to silk. It is in this form, which has been commonly utilized in hosiery since the 1940s, that nylons are perhaps most familiar. In addition to their luxurious appearance, fine nylon materials have a low moisture absorbency and are resistant to mold, mildew, chemicals, and abrasion.