Polyester is a category of polymers that contain one or two ester linkages in every repeat unit of their main chain. As a specific material, it most commonly refers to a type called polyethylene terephthalate (PET). Polyesters include some natural products, such as those found in plants and insects. Natural polyesters and a few synthetic ones are biodegradable, but most commercial polyesters are not. Polyesters are used extensively in clothing.