Editing 19-inch rack (section)

== Overview and history ==
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 cabinet''', '''rack-mountable''', or occasionally simply '''shelf'''. The height of the electronic modules is also standardized as multiples of {{convert|1.75|in|mm|sigfig=4}} or one [[rack unit]] or U (less commonly RU). The industry-standard rack cabinet is 42U tall;<ref>{{cite book |last1=West |first1=Jill |last2=Dean |first2=Tamara |last3=Andrews |first3=Jean |date=2015 |title=Network+ Guide to Networks |chapter=Structured Cabling and Networking Elements |chapter-url=https://books.google.com/books?id=KjadBAAAQBAJ&pg=PA169 |location=Boston, MA |publisher=Engage Learning |page=169 |isbn=9781305480865 |edition=Seventh |access-date=9 December 2019 |quote=Racks are measured in rack units (RU or U) with the industry standard being 42U tall — about 6 feet.}}</ref> however, many data centers have racks taller than this.<ref>{{cite web | url=https://www.datacenterknowledge.com/servers/data-center-racks-getting-taller-wider-deeper | title=Data Center Racks Getting Taller, Wider, Deeper }}</ref>

The term '''relay rack''' appeared first in the world of [[telephony]].<ref>{{cite journal |last=Lowenthal |first=Max |date=February 2, 1899 |title=The New Exchange of the Central New York Telephone and Telegraph Co. at Syracuse, N.Y. |url=https://books.google.com/books?id=bH5NAAAAYAAJ&pg=RA2-PA144 |journal=The Electrical Engineer |volume=XXVII |issue=561 |pages=142–147 |access-date=9 December 2019 }}</ref>

By 1911, the term was also being used in [[railroad signaling]].<ref>{{cite magazine |author=<!--Staff writer(s); no by-line.--> |title=New Electric Interlocking at Allentown, PA |url=https://books.google.com/books?id=z8w0AQAAMAAJ&pg=PA344 |magazine=The Signal Engineer |location=Chicago |publisher=The Railroad Gazette |date=September 1911 |volume=4 |issue=9 |pages=344–345|access-date=9 December 2019 }}</ref> There is little evidence that the dimensions of these early racks were standardized.

[[file:Rack_mounted_telephone_equipment_(1923).png|thumb|right|upright|Telephone equipment racks (1923)]]
The 19-inch rack format with rack-units of {{convert|1.75|in|mm|2}} was established as a standard by AT&T around 1922 in order to reduce the space required for [[repeater]] and termination equipment in a telephone company [[Central office (telecommunications)|central office]]. 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 custom-made racks, one per repeater. But in light of the rapid growth of the toll network, the engineering department of AT&T undertook a systematic redesign, resulting in a family of modular factory-assembled panels all "designed to mount on vertical supports spaced 19{{frac|1|2}} inches between centers. The height of the different panels will vary,... but... in all cases to be a whole multiple of {{frac|1|3|4}} inches."<ref>{{cite magazine |last=Demarest |first=Charles S. |date=July 1923 |title=Telephone Equipment for Long Cable Circuits |url=https://archive.org/details/bstj2-3-112 |magazine=Bell System Technical Journal |volume=2 |issue=3 |location=New York |publisher=American Telephone and Telegraph Company |pages=112–140 |access-date=9 December 2019 }}</ref>

By 1934, it was an established standard with holes [[Tap and die|tapped]] for [[Unified Thread Standard|12-24 screws]] with alternating spacings of {{convert|1.25|in|mm|2}} and {{convert|0.5|in|mm|2}} <ref>{{cite magazine |last=Mezger |first=G. Robert |date=November 1934 |title=The Relay Rack in Amateur Construction |magazine=QST |publisher=American Radio Relay League |pages=27–30 }}</ref> The [[List of EIA standards#300–399|EIA standard]] was revised again in 1992 to comply with the [[Omnibus Foreign Trade and Competitiveness Act|1988 public law 100-418]], setting the standard U as {{convert|15.875|mm|in|3|abbr=on}} + {{convert|15.875|mm|in|3|abbr=on}} + {{convert|12.7|mm|in|3|abbr=on}}, making each ''U'' {{convert|44.45|mm|in|2}}.<ref>ANSI/EIA-310-D-1992</ref>

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 {{convert|19|in|mm|1|adj=on}} standard rack arrangement is widely used throughout the [[Telecommunications industry|telecommunications]], [[computer|computing]], [[Sound recording and reproduction|audio]], [[video]], [[entertainment]] and other industries, though the [[Western Electric]] [[23-inch rack|23-inch standard]], with holes on {{convert|1|in|mm|1|adj=on}} centers, is still used in legacy [[ILEC]]/[[Competitive local exchange carrier|CLEC]] facilities.

Nineteen-inch racks in two-post or four-post form hold most equipment in enterprise [[data centers]], [[ISP]] facilities, and professionally designed corporate [[server room]]s, although [[hyperscale computing]] typically use wider racks.<ref>{{Cite web |title=Open Compute Project |url=https://www.opencompute.org/projects/rack-and-power |access-date=2024-03-01 |website=Open Compute Project |language=en}}</ref><ref>{{Cite book |last1=Barroso |first1=Luiz André |url=https://books.google.com/books?id=b951DwAAQBAJ |title=The Datacenter as a Computer: Designing Warehouse-Scale Machines, Third Edition |last2=Hölzle |first2=Urs |last3=Ranganathan |first3=Parthasarathy |date=2018-10-29 |publisher=Morgan & Claypool Publishers |isbn=978-1-68173-434-7 |language=en}}</ref> They allow for dense hardware configurations without occupying excessive floor space or requiring shelving.

[[File:Tektronix oscilloscope 7603 in 19 inch rack version with three plug-ins.jpg|thumb|right|350px|A professional-grade [[oscilloscope]] [[Tektronix analog oscilloscopes|Tektronix 7603]] (1970s) for use in electronics and scientific laboratories. Typically for rack-mounted devices, the width of the front panel exceeds the width of the device itself, which provides the overlap zone (including screw holes) with the left and right rack rails.]]
Nineteen-inch racks are also often used to house professional audio and video equipment, including [[amplifier]]s, [[effects unit]]s, interfaces, [[headphone]] amplifiers, and even small-scale audio mixers. A third common use for rack-mounted equipment is [[industrial control system|industrial power, control, and automation hardware]].

Typically, a piece of equipment being installed has a front panel height {{convert|1/32|in|in mm|sigfig=2}} less than the allotted number of Us.  Thus, a 1U rackmount computer is not {{convert|1.750|in|mm}} tall but is {{convert|1.719|in|mm}} tall.  If ''n'' is number of rack units, the ideal formula for panel height is {{nowrap|1=''h'' = 1.75''n'' − 0.031}} for calculating in inches, and {{nowrap|1=''h'' = 44.45''n'' − 0.794}} for calculating in millimeters. This gap allows a bit of room above and below an installed piece of equipment so it may be removed without binding on the adjacent equipment.
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