U is the standard size of a rack-mountable piece of equipment as defined by the Electronic Industries Alliance (EIA). Also known as a rack unit (RU), rack-space or "shelf," a 1-U device has a 17-¾-inch-width body with a 19-by-1 3/4-inch-high face. The additional 1 ¼ inch of width on the face is equally distributed to each side and is drilled with holes. The piece is mounted by bolts that pass through these holes into a metallic track that has holes drilled in an alternating pattern every 1 ¾ inch on the vertical interior of the case.
The U measurement indicates how many standard size pieces of gear can fit in a case or the size of a piece of equipment. For instance, a graphic equalizer may only be 1 U (19-by-1 3/4 inch) while a power amp could be 4 U ( the face of the amp 19-by-6 4/5 inch).
The first 19-inch rack cabinet systems were used for electric switches and relays in the telecommunication industry. By the 1980s, the music industry began rack mounting equipment. Unfortunately, some DJ and recording studio rack-mountable equipment can fail to meet EIA standards. If you purchase a piece of equipment with a specific purpose that requires it to be rack mounted (such as a mobile DJ or live P.A. rig), take this into consideration.