The term rack often refers to the EIA-standardized 19-inch mounting format, used in such places as computer datacenters and music studios, for consolidating many pieces of electronic equipment in a small space. Many other standards do exist, especially in the telecommunications world – the EIA also provides 23-inch and 35-inch rack standards – but the 19-inch format is prevalent enough that when you say "19-inch rack," most everyone understands what you mean.

As its name implies, the 19-inch EIA standard calls for a faceplate 19 inches wide, with rails on either side having mounting holes 18.31 inches on center. The basic unit of rack space is, appropriately enough, the unit (also called the U), which in the EIA standard is 1.75 vertical inches (with two holes in each rail, 1.25 inches on center). Most racks also have a third hole at the vertical center of the unit.

Used in a music studio, racks may hold audio signal processing devices such as compressors or effects units, mixers, amplifiers, or synthesizer modules controlled by external MIDI interfaces. Some of these are purpose-built for use in a rack, while others may simply be boxes with removable "ears" at the front corners to make the device rack-mountable.

In the IT datacenter, two major types of racks are common:

  1. A telco rack or relay rack consists of a free-standing pair of rails, usually bolted to the floor and/or ceiling, with the normal pattern of mounting holes. These are mostly used for network equipment like routers, hubs and switches, and patch panels for connecting cables to other locations.
  2. The server rack may contain these items as well, but as one would expect, it is mostly used for servers – and associated equipment such as a KVM switch for operating multiple computers with a single keyboard, mouse and video display. These racks are usually enclosed, with doors and side panels, and have two sets of rails, front and rear (though the depth is not standardized and varies by manufacturer). Again, the servers may be tower-style units adapted for rack use by mounting kits, or special chassis designed for high-density server farm deployments. Such computers, especially those with a height of one unit (1U), are often called pizza boxes.