Rack screws are used to attach equipment to the vertical uprights of a 19" rack. Racks do not have a standard thread type. There are three common thread types for rack screws: 10-32 12-24 and M6. Since the introduction of the square hole rack and Cage Nuts the exact thread can be selected and changed at will. 12-24 is now more common than 10-32 in pre-threaded racks. 12-24 fasteners are slightly larger than 10-32 and have courser threads. The term 12-24 comes from Unified and American Screw Threads for Bolts Nuts and Machine Screws standards published by ANSI B1.1-1974. The number "12" is simply a size designator corresponding to the nominal diameter of the bolt. The number "24" refers to 24 threads per inch. You can identify a 12-24 screw by measuring the diameter with a ruler at slightly less than 7/32" (slightly larger than 3/16" or 0.210").