The unsaturated polyesters are extremely versatile in terms of the forms in which they are used. Polyester resins can be formulated to be brittle and hard, tough and resilient, or soft and flexible. In combination with reinforcements such as glass fibers, they offer outstanding strength, a high strength-to-weight ratio, chemical resistance, and other excellent mechanical properties. The two dominate materials in this plastics family are polyethylene terphthalate (PET) and polybutylene terephthalate (PBT). The thermoplastic polyesters are similar in properties to types 6 and 6/6 nylons, but have lower water absorption and higher dimensional stability than the nylons.