Core-spun yarns have a central core that is wrapped with staple fibres, and are produced in a single operation at the time of spinning. Core spun, or “polycore”, yarn is created by twisting staple fibers around a central filament core, usually made of polyester for extra strength. It is 40% to 50% stronger than normally spun yarn of the same weight, and reduces the number of broken stitches when sewing seams and hems on denim.