Equipment mounted in several 19-inch racks
A 19-inch rack is a standardized (EIA 310-D, IEC 60297 DIN 41494 SC48D) system for mounting various electronic modules in a "stack", or rack. Because of their origin as mounting systems for telephone switching equipment, they are sometimes called relay racks. The rack consists of two parallel thin metal plates standing vertically with a space of approximately 18 inches between them. They have holes drilled through them at regular intervals, so that the centrelines of the holes in the two strips are 18.3 inches apart. Think of a 2 inch wide door frame around an 18 inch wide door. In this case the two plates are positioned similarly to the vertical posts of the door frame. A second set of similar plates sometimes stands behind the first, 20 or 26 inches away. The resulting rack looks like the four vertical sides of a large box, enclosing a rectangle 18 by 20 inches – the extra inch across the front is to the mounting holes.
Equipment designed to be placed in a rack is typically described as rack-mount, a rack mounted system, a rack mount chassis, subrack, or occasionally, simply shelf. The vertical height of 19-inch rack equipment is usually a multiple of a standard unit of height called the 'U', meaning Unit. One U is 1.75 inches (44.45 mm).
With a vertical rack the weight is borne entirely by the front and rear plates, which presents a problem when trying to remove a chassis because all four corners have to be removed at the same time. The common solution is to use a set of sliders mounted between the front and rear racks, into which the chassis is slid towards the rear. These systems typically include holes for screwing the front plate into the rack.
This standard rack arrangement is widely used throughout the telecommunication, computing, audio and video industries, as well as others.