A 19-inch rack is a standardized (EIA 310-D, IEC 60297 and DIN 41494 SC48D) system for mounting various electronic modules in a "stack", or rack, 19 inches (482.6 mm) wide. Equipment designed to be placed in a rack is typically described as rack-mount, a rack mounted system, a rack mount chassis, subrack, or occasionally, simply shelf. The slang expression for a subrack (generally 1U = 1.75 in = 44.45 mm height) is "pizza box" due to the similarity in size and shape.

Because of their origin as mounting systems for railroad signaling relays, they are still sometimes called relay racks, but the 19-inch rack format has remained a constant while the technology that is mounted within it has changed to completely different fields. This standard rack arrangement is widely used throughout the telecom, computing, audio, entertainment, and other industries.

The racks mounting fixture consists of two parallel metal strips (also referred to as "rails" or "panel mount") standing vertically. The strips are each 0.625 inches (15.875 mm) wide, and are separated by a gap of 17.75 inches (450.85 mm), giving an overall rack width of 19 inches (482.6 mm). The strips have holes in them at regular intervals, with both strips matching, so that each hole is part of a horizontal pair with a center-to-center distance of 18.3 inches (464.82 mm).

The holes in the strips are arranged vertically in repeating sets of three, with center-to-center separations of 0.5 inch (12.7 mm), 0.625 inch (15.875 mm), 0.625 inch (15.875 mm). The hole pattern thus repeats every 1.75 inches (44.45 mm). Racks are divided into regions, 1.75 inches in height, within which there are three complete hole pairs in a vertically symmetric pattern, the holes being centered 0.25 inch (6.35 mm), 0.875 inch (22.225 mm), and 1.5 inch (38.1 mm) from the top or bottom of the region. Such a region is commonly known as a "U", for "unit", and this unit measures heights within racks.

Equipment Mounting
Originally, the mounting holes were tapped to receive a particular type of threaded bolt. These are no longer typically used for frequently changed server racks rendering the mounting hole unusable. Tapped-hole racks are still used for hardware that rarely changes, such as phone or network cabling panels and relay racks.

The tapped-hole rack was first replaced by round-hole racks. The holes are large enough to permit a bolt to be freely inserted through without binding, and bolts are fastened in place using cage nuts. A cage nut consists of a spring steel cage, designed to clip onto the open mounting hole, within which is a captive nut.

The next innovation in rack design has been the square-hole rack. Square-hole racks allow boltless mounting, such that the rack-mount equipment only needs to insert through and hook down into the lip of the square hole. Installation and removal of hardware in a square hole rack is very easy and boltless. Older equipment meant for round-hole or tapped-hole racks can still be used, with the use of cage nuts made for square-hole racks.

Rack-mountable equipment is mounted simply by bolting its front panel to the rack, or with a square-holed rack by clipping or some other variation on the theme. Having all the structural support at one edge of the equipment is a weakness of this system, and so heavier equipment is designed to use a second pair of mounting strips located at the back of the equipment.

The strength required of the mounting strips means they are invariably not merely flat strips but actually a wider folded strip arranged around the corner of the rack. The strips are usually made of steel of around 2 mm thickness, or of slightly thicker aluminium.

Heavy equipment, for which attaching or detaching at all four corners simultaneously would pose a problem, is often not mounted directly onto the rack but instead is mounted via rails. When in place, the equipment may also then be bolted to the rack. The rails may also be able to fully support the equipment in a position where it has been slid clear of the rack.

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