Despite their name, professional grade "acetate" discs do not contain any acetate. They consist of an aluminum disc with a coating of nitrocellulose lacquer. Glass was often used for the substrate during World War II, when aluminium was in short supply. The production process results in a disc that is different in several ways from the vinyl records sold to customers at retail. Most noticeably, vinyl records are comparatively lightweight and flexible, while acetates are usually rigid and considerably heavier because of their metal cores.