Nylon is any of a group of polymers containing amide groups recurring in the chain (polyamides). The commonest nylon is made by condensation of adipic acid and hexamethylene diamine. Nylon is chemically inert, heat-resistant, tough, elastic, and very strong, and is extruded and drawn to make synthetic fibers, filaments, and bristles, rolled out as sheets, or cast and molded into bearings, gears, zippers, etc. Developed in the US in the 1930s, it is used chiefly in yarn, cordage, and molded products. Hard and tough or soft and rubbery nylon products can be made by varying their composition.