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) and holes tapped for 12-24 screws with alternating spacings of 1.25 inches (31.75 mm) and 0.5 inches (12.70 mm) was an established standard by 1934. 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.
A 19-inch rack used for switches at the DE-CIX in Frankfurt, Germany
Nineteen-inch racks in two-post or four-post form hold most equipment in modern data centers, ISP facilities, and professionally designed corporate server rooms. They allow for dense hardware configurations without occupying excessive floorspace or requiring shelving.
Nineteen-inch racks are also often used to house professional audio and video equipment, including amplifiers, effects units, interfaces, headphone amplifiers, and even small scale audio mixers. A third common use for rack-mounted equipment is industrial power, control, and automation hardware.
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