Rack unit

The standard was adopted worldwide as IEC 60297 Mechanical structures for electronic equipment – Dimensions of mechanical structures of the 482.6 mm (19 in) series, and defines the sizes for rack, subrack (a shelf-like chassis in which cards can be inserted), and the pitch of printed circuit boards/cards providing physical compatibility of technological equipment, typically in telecommunications.

To allow space between adjacent rack-mounted components, a panel is 1⁄32 inches (0.03125 in or 0.794 mm) less in height than the full number of rack units would imply.

Universal square holes are becoming the most common as these allow the insertion of replaceable cage nuts for the type of thread needed.

It can also describe a unit that is 1U high and half the depth of a 4-post rack (such as a network switch, router, KVM switch, or server), such that two units can be mounted in 1U of space (one mounted at the front of the rack and one at the rear).

This "half rack width" concept is popular in applications where IT equipment is being used by military who are unable to use traditional 1U full-depth IT appliances due to their large size.