Nonmetallic materials cover three broad categories, namely plastics, composites, and ceramics. In many applications, considerable overlap occurs between plastics and composites, because the composite matrix is a plastic material. The overlap between composites and ceramics usually centers on the use of glass fibers. The terms polymer, plastics, polymers, resins, rubber, and elastomer are often confused. Polymers are organic compounds that come from natural sources or are synthetic. The word polymer means many (poly) mers. Mers are small units, generally simple organic molecules. Crude oil and natural gas supply the chemicals required for the production of plastics. The possible combinations of monomers to create polymers are virtually endless. However, three possibilities are most common: homopolymers, copolymers, and terpolymers. Thermosets are plastics containing molecules that are chemically linked to each other.