Rack-mount

A 19-inch rack is a standardized frame or enclosure for mounting multiple electronic equipment modules. Each module has a front panel that is 19 inches (48.3 cm) wide. The 19-inch dimension includes the edges, or "ears", that protrude on each side which allow the module to be fastened to the rack frame with screws. Common uses include server, audio, and scientific lab equipment.

Equipment designed to be placed in a rack is typically described as rack-mount, rack-mount instrument, a rack mounted system, a rack mount chassis, subrack, rack mountable, or occasionally simply shelf. The height of the electronic modules is also standardized as multiples of 1.752 inches (44.50 mm) or one rack unit or U (less commonly RU). The industry standard rack cabinet is 42U tall.

The term relay rack appeared first in the world of telephony. By 1911, the term was also being used in railroad signaling. There is little evidence that the dimensions of these early racks were standardized.

The 19-inch rack format with rack-units of 1.75 inches (44.45 mm) was established as a standard by AT&T around 1922 in order to reduce the space required for repeater and termination equipment for toll cables.

The earliest repeaters from 1914 were installed in ad-hoc fashion on shelves, in wooden boxes and cabinets. Once serial production started, they were built into bespoke racks, one per repeater. But in light of the rapid growth of the toll network, a systematic redesign was undertaken by the engineering department of AT&T, resulting in a family of modular factory assembled panels all "designed to mount on vertical supports spaced 19½ inches between centers. The height of the different panels will vary, […] but […] in all cases to be a whole multiple of 1¾ inches."

By 1934, it was an established standard with holes tapped for 12-24 screws with alternating spacings of 1.25 inches (31.75 mm) and 0.5 inches (12.70 mm) The EIA standard was revised again in 1992 to comply with the 1988 public law 100-418, setting the standard U as 15.9 mm (0.63 in) + 15.9 mm (0.63 in) + 12.7 mm (0.50 in), making each "U" officially 1.752 inches (44.50 mm).

The 19-inch rack format has remained constant while the technology that is mounted within it has changed considerably and the set of fields to which racks are applied has greatly expanded. The 19-inch (482.6 mm) standard rack arrangement is widely used throughout the telecommunication, computing, audio, video, entertainment and other industries, though the Western Electric 23-inch standard, with holes on 1-inch (25.4 mm) centers, is still used in legacy ILEC/CLEC facilities.

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