A system of classification of steels and steel alloys, by their chemistry. It was developed by the Society of Automotive Engineers (SAE). It was later adopted and expanded by the American Iron and Steel Institute (AISI). Both plain-carbon and low-alloy steels are identified by a four-digit number. The first number indicates the major alloying elements and the second, a subgrouping of the major alloy system. The last two digits indicate the approximate carbon content of the metal.