A 23-inch rack is used for housing telephone

Telephone

The telephone , colloquially referred to as a phone, is a telecommunications device that transmits and receives sounds, usually the human voice. Telephones are a point-to-point communication system whose most basic function is to allow two people separated by large distances to talk to each other...

 (primarily), computer

Computer

A computer is a programmable machine designed to sequentially and automatically carry out a sequence of arithmetic or logical operations. The particular sequence of operations can be changed readily, allowing the computer to solve more than one kind of problem...

, audio

Sound

Sound is a mechanical wave that is an oscillation of pressure transmitted through a solid, liquid, or gas, composed of frequencies within the range of hearing and of a level sufficiently strong to be heard, or the sensation stimulated in organs of hearing by such vibrations.-Propagation of...

 and other equipment though is less common than the 19-inch rack

19-inch rack

A 19-inch rack is a standardized frame or enclosure for mounting multiple equipment modules. Each module has a front panel that is wide, including edges or ears that protrude on each side which allow the module to be fastened to the rack frame with screws.-Overview and history:Equipment designed...

. The size notes the width of the faceplate for the installed equipment. The rack unit

Rack unit

A rack unit or U is a unit of measure used to describe the height of equipment intended for mounting in a 19-inch rack or a 23-inch rack...

 is a measure of vertical spacing and is common to both the 19 and 23 inches (584.2 mm) racks.

Hole spacing is either on 1 inches (25.4 mm) centres (Western Electric standard), or the same as for 19 inches (482.6 mm) racks (convert|0.625|in|abb=on|disp=/}} spacing).

Another name for this type of racks is ETSI rack, relating to the European Telecoms Standards Institute.