A condensation polymer formed by the interaction of polyhydric alcohols and polybasic acids. Linear polyesters are saturated thermoplastics and linked by dipole-dipole attraction as the carbonyl groups are polarized. They are extensively used as fibres (e.g. Terylene). Unsaturated polyesters readily copolymerize to give thermosetting products. They are used in the manufacture of glass-fibre products. See also alkyd resin.