1U/2U/3U/4U

A rack unit is a unit of measure used to describe the height of a server or other similar device like storage system mounted in a 19-inch rack. One rack unit is 44.45 mm (1.75 in). One rack unit is commonly designated as "1U"; similarly, 2 rack units are "2U" and so on. The size of a server is usually described as a number in "U". Professional audio and video gears often come with rack mount options and use the same measurement specifications. Half Rack units typically describe units that fit in a certain number of RU, but occupy only half the width of the rack (9.5"). For example, a "4U Half Rack" DVCAM deck would occupy 4U space (1.75 * 4) height x 9.5" width. In theory, two half rack decks could occupy the 4U space. Interestingly, a rack unit is equal to Vershok, which is an obsolete Russian length measure unit.