SRS-What is a "U"

Because of their origin as mounting systems for railroad signaling relays, they are still sometimes called relay racks, but the 19-inch rack format has remained a constant while the technology that is mounted within it has changed to completely different fields. This standard rack arrangement is widely used throughout the telecommunication, computing, audio, entertainment and other industries, though the Western Electric 23-inch standard, with holes on 1-inch centers, prevails in telecommunications.

Typically, a piece of equipment being installed has a front panel height 1/32-inch (.031") less than the allotted number of Us. Thus, a 1U rackmount computer is not 1.75-inches tall but is 1.719 inches (43.7 mm) tall. 2U would be 3.469 inches (88.1 mm) instead of 3.5-inches. 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.