19 inch rack mounts originate from the mounting systems for railroad signalling relays, which is why they are also called relay racks. However 19 inch rack mount has become a constant for mounting any form of electronic technology for a wide range of different industries.

The standard 19 inch rackmount arrangement is now widely used in telecommunication, computing, audio, entertainment and other industries.

19 inch rack mounts are used to house professional audio and audio visual equipment. The type of equipment that is often housed within 19 inch racks includes amplifiers, effects units, interfaces, audio mixers, servers, CD, MP3 and DVD players.

Racks are widely used to house computer server equipment. Large racks allow dense hardware configurations to be arranged without occupying excessive floor space. It is these large computer servers which will be 40 to 46U high.

A rack unit is a measurement used to describe the height in a rack. One rack unit (1U) is equivalent to 1.75" (44.45mm) in height. For example 2U = 3.5".

These U's are referred to when choosing rack equipment, because devices such as rack drawers and rack shelves will be certain U heights e.g. 2U, 3U, 4U etc.