Several steel numbering systems are in general use. The ASTM, AISI, and SAE have devised codes to define the alloying elements and carbon content of steels. The following table lists some of the AISI/SAE designations for commonly used steel alloys. The first two digits indicate the principal alloying elements. The last two digits indicate the amount of carbon present, expressed in hundredths of a percent. ASTM and the SAE have developed a new Unified Numbering System for all metal alloys, which uses the prefix UNS followed by a letter and a 5-digit number. The letter defines the alloy category, F for cast iron, G for carbon and low-alloy steels, K for special-purpose steels, S for stainless steels, and T for tool steels. For the G series, the numbers are the same as the AISI/SAE designations with a trailing zero added. For example, SAE 4340 becomes UNS G43400.