Covers screws in diameters from M1.6 to M20. It applies to countersunk, raised countersunk, pan and cheese head types, which are in accordance with international agreement, and to an indented hexagon head type, the across-corners and across-flats dimensions of which are in accordance with international agreement but for which the head height has been increased in order to provide better wrenching capabilities. In addition, a mushroom and an integral hexagon washer head type are included; these heads are not in accordance with any international agreement, but are based on current imperial practices. Mechanical properties are given with respect to steel, brass and aluminium alloy machine screws; the steel screws are of strength grade 4.8. Only the ISO first choice diameter series are given in the body of the specification; the ISO second choice diameter series is given in an appendix. Other appendices give preferred diameter/length combinations, dimensions of 'pilot point ends' for high volume production process, and gauging requirements for both 'Pozidriv' and 'Phillips' cross-recesses.