The product of polymerization of amino acids or the condensation of a diamine with a dicarboxylic acid. Polyamides are a class of polymers in which individual structral units are joined by amide bonds (R–CO–NH₂). Whitish, translucent, and high-melting polyamide resins can be essentially transparent and amorphous when their melts are quenched. On cold drawing and annealing, most become quite crystalline and translucent. However, some polyamides based on bulky repeating units are inherently amorphous. The polymers are used for fibers, bristles, bearings, gears, molded objects, coatings, and adhesives. The term nylon formerly referred specifically to synthetic polyamides as a class. Because of many applications in mechanical engineering, nylons are considered engineering plastics.