IEC metric screw sized connectors is a family of electrical connectors defined by IEC that are named according to their ISO metric screw thread, namely M5, M8 and M12. The number gives their outer screw thread diameter in millimeters as with the identically named screws. However, the connectors are further classified by a so-called coding, denoted by one or more letters, which defines things like pin layout, shape of connecting surfaces and electrical properties.