Most professional server and computer gear in general conforms to a 19-inch rack specification that has been standardized by a variety of organizations, including the Electronic Industries Alliance (EIA 310-D), and the Deutsches Institut für Normung (DIN 41494). The latter organization is where MIDI cables get their other name: the 5-pin DIN. It is the same standard that has been used for decades in telecommunications, computing, and even consumer electronics.
Rack Mount Gear
Rack-mount gear is any equipment that is designed for mounting and use within a computer rack. Sometimes rack-mount gear will have telltale rack “ears” (flanges) pre-installed or built into the device, and sometimes these rack ears will be available as an option. In some cases, a piece of equipment might be designed so that it can be fastened onto a rack tray or shelf. The shelf, which has the appropriate mounting flanges built in, can then be installed into a rack. Sometimes equipment mounted onto a rack shelf ends up slightly taller than it should be which can make for some tight fits.
The height of rack-mount gear is measures in increments of 1.75 inches (4.45 cm), and the depth varies widely. Most rack-mount servers and telecommunications devices can have depths of 33 inches (83.82 cm) or more. A good rule of thumb when trying to figure out how deep of a cabinet to build is to measure the depth (from front to back) of the deepest piece of gear, and then add about 4 to 6 inches (10.16 to 15.24 cm) to allow room for cables and power supplies.
The Rack Unit (or Modular Unit)
A rack unit is a measure of the height of a piece of rack-mount gear. One unit is 1.75 inches (4.45 cm). The term “rack unit” can also be used to describe the actual piece of equipment that you intend to mount in the rack. A rack unit is often abbreviated with “U” and sometimes “RU”, so a 1U effects processor is 1.75 inches (4.45 cm) high. Remember: The unit of measure says nothing about the width of the gear (19 inches [48.26 cm] in the case of most audio gear) or its depth, which can vary widely.
Rack Rails & Screws
Rack rails are the hardware used to hold rack-mount gear in place within an equipment rack. Though rails can be made of just about any material (including wood or plastic), they're usually made from 11-gauge steel, which is about 0.125 inches (0.318 cm) thick, and bent to form an L shape. The side that holds the gear is typically 0.625 to 0.75 inch (1.59 to 1.91 cm) wide, and the side that gets mounted to the inside of the cabinet is typically about 1.25 inch (3.18 cm) wide. Steel rack rails have a set of mounting holes that usually conform to the EIA standard so that they match up nicely with the holes (or notches) in the mounting ears of rack-mount gear. The distance from one hole to the next is the repeating set of 5/8 inch (1.59 cm), 5/8 inch (1.59 cm), and 1/2 inch (1.27 cm), on center, and the first hole is 1/4 inch (0.64 cm) from each end of the rail.
The mounting holes in steel rack rails should be pre-tapped (pre-threaded) for 10-32 machine screws. The “10” (or #10, spoken “number ten”) is the common name for the screw, and the “32” means that it has 32 threads per inch. You might also see the acronym UNF used to describe these screws (e.g., #10 UNF) – this just means that it's a fine thread as opposed to a coarse thread. View rack screws and cage nuts here.
Mounting the Rails
There are a number of different ways to mount rails to a cabinet. One of the most solid methods is to use sheet metal screws, which (unlike wood screws) are threaded all the way up to the head. Depending on what your cabinet is made of e.g. 3/4" plywood, you can get away with 3/4" screws because the thickness of the rails will prevent the screws from breaking through the wood. For most racks, it us recommended that you use one screw for every 2U, but no less than two screws per rail.