Equipment designed to be placed in a rack is typically described as rack-mount, a rack mounted system or a rack mount chassis. The vertical depth of 19-inch rack equipment is usually a multiple of a standard unit of height called the 'U', meaning Unit, whose dimensions were originally developed for a horizontal rack mounting system known as Eurocard. A 1 U is 1.75 inches (44.45 mm), 1U, 2U and 3U are the most common sizes.
In the original Eurocard system the cards were mounted vertically, standing on end in a 18" long slot. This meant the weight of the equipment rested on the bottom of the horizontal rack. 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, although this is somewhat vestigial.