 The "M" designation for metric screws indicates the nominal outer diameter of the screw thread, in millimetres (e.g., an M6 screw has a nominal outer diameter of 6 millimetres).The design principles of ISO general-purpose metric screw threads ("M" series threads) are defined in international standard ISO 68-1.Each thread is characterized by its major diameter, "D" ("D"maj in the diagram), and its pitch, "P".