The American Iron and Steel Institute (AISI) has a standard that is widely accepted in the United States and other countries. However it does not cover all metal*lss and, in many instances, it does not convey enough information to speak to all of the attributes of some metal*lss. Therefore two United States standards organizations, ASTM (American Society for Testing and Metals) and SAE (Society of Automotive Engineers), have developed a standard for classification of metal*lss that is independent of AISI standards, the UNS. It is composed of seventeen letters that represent seventeen metal*ls types. Each letter is followed by five digits that signify the specific makeup of each of these metal*lss types. The first three or four digits are derived from different standards.

List of UNS metal*lss designations (only those underlined are important to machinists)

• AXXXXX Aluminum
• CXXXXX Copper and copper alloys
• EXXXXX Rare earth and rare earth like (cerium, actium, etc.)
• FXXXXX Cast iron
• GXXXXX Carbon steel
• LXXXXX Low melting point alloys (lead, cadmium, mercury, etc.)
• MXXXXX Misc. non-ferrous metal*lss alloys (antimony, arsenic, uranium, etc.)
• NXXXXX Nickel and nickel alloys
• PXXXXX Precious metal*lss (gold, silver, etc.)
• RXXXXX Reactive and refractory metal*lss and alloys (boron, vanadium, etc.)
• SXXXXX Stainless steel
• TXXXXX Tool Steel
• WXXXXXWelding filler materials
• ZXXXXX Zinc and zinc alloys