Editing 19-inch rack (section)

===Fastening===
Originally, the mounting holes were [[taps and dies|tapped]] with a particular screw thread. When rack rails are too thin to tap, [[rivet nut]]s or other [[threaded insert]]s can be used, and when the particular class of equipment to be mounted is known in advance, some of the holes can be omitted from the mounting rails.<ref>[http://www.cs.uiowa.edu/~jones/pdp8/UI-8/guide.shtml#pdp8rack The Computer Rack] section of [http://www.cs.uiowa.edu/~jones/pdp8/UI-8/guide.shtml The University of Iowa's DEC PDP-8], documents a relay rack made in 1965; Nov. 2011.</ref>

Threaded mounting holes in racks where the equipment is frequently changed are problematic because the threads can be damaged or the mounting screws can break off; both problems render the mounting hole unusable. Tapping large numbers of holes that may never be used is expensive; nonetheless, tapped-hole racks are still in use, generally for hardware that rarely changes. Examples include telephone exchanges, network cabling panels, broadcast studios and some government and military applications.

The tapped-hole rack was first replaced by clearance-hole (Round Hole, Round Unthreaded Holes,<ref>{{cite web|url=http://www.server-racks.com/rack-hole-types.html|title=Define: Rack Hole Types|website=server-racks.com|date=14 October 2007 }}</ref> and Versa Rail<ref>{{cite web|url=http://www.server-racks.com/dell-versa-rail.html|title=What is a Versa Rail|website=server-racks.com|date=15 October 2007|archive-url=https://web.archive.org/web/20221129080420/https://www.server-racks.com/dell-versa-rail.html|archive-date=2022-11-29|url-status=dead}}</ref>) 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 nut]]s. In the event of a nut being stripped out or a bolt breaking, the nut can be easily removed and replaced with a new one. Production of clearance-hole racks is less expensive.

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, where the weight of the equipment and small retention clips are all that is necessary to hold the equipment in place. 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.