What mounting hardware do I need? It depends on the mounting rails of the rack enclosure or relay rack you will be ordering. Most 4-Post server racks, cabinets, LAN enclosures either use Cage Nuts and Screws for square hole type Universal Mounting Rails or 1032 tapped screws for round hole style EIA Standard Mounting Rails. Please be aware that almost all 2-post open relay racks use 1032 Tapped Screws (round hole mounting rails).
10-32 Tapped Cage Nuts and Screws - American Version - Commonly used in all rack mount applications including music, video, broadcast, data and more. The "10" refers to the drill size for a tapped (threaded) hole. The outside diameter of a 10-32 screw is 0.19", it is smaller than a 12-24 screw. This screw type has 32 threads per inch.
12-24 Tapped Screws - American Version - The "12" refers to the drill size required for a tapped (threaded) hole (a #12 drill is 0.189"). The outside diameter of a 12-24 screw is 0.2160". It is larger than a 10-32 screw. This screw type has 24 threads per inch
M6 Tapped Cage Nuts and Screws - Metric Version - Metric thread size of 6 millimeters. Typical thread size for European rack applications. Also used in Compaq racks and Euro racks sold here in the US. Larger than both 10-32 and 12-24.