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. 10-32 threads are historically the original rack screw type. The term 10-32 comes from Unified and American Screw Threads for Bolts Nuts and Machine Screws standards published by ANSI B1.1-1974. The number "10" is simply a size designator corresponding to the nominal diameter of the bolt. The number "32" refers to 32 threads per inch. You can identify a 10-32 screw by measuring the diameter with a ruler at exactly 3/16" (0.190").