Iron mixed with a small amount of carbon.
Swiss watchmakers collectively refer to a watch's steel components (screws, index etc.) as les aciers (French for steel). Among the different grades of steel, semi-hard is used for transmission arbors and stamped or bent parts. Hard steel is used for screws, pinions and parts that do not require very hard grade steel. Very hard and extra hard steel is used, for example, for springs and files. The majority of bracelets (straps) and cases use 316L surgical steel for its anallergic properties.