telephone equipment were generally high. A series of studies led to the adoption of frames high, with modular widths in multiples of —most often wide. ETSI rack The ETSI rack is defined by the European Telecommunications Standards Institute (ETS 300 119). The distance of the right edge of the right mounting rail to the left edge of the left mounting rail is . As 535 mm is very close to 21 inches, these racks are sometimes called 21-inch racks. The gap between the posts is . As 19-inch equipment has a maximum width of , they can easily be mounted in an ETSI rack by means of an ETSI bracket or adapter plate. In contrast to the "19-inch world", ETSI also defined the size of the rack enclosure: the four allowed widths are and two allowed depths are . Hole spacing is . 23-inch rack A rack is used for housing telephone (primarily), computer, audio, and other equipment though is less common than the 19-inch rack. The size denotes the width of the faceplate for the installed equipment. The rack unit is a measure of vertical spacing and is common to both the 19 and 23 inch racks. Hole spacing is either on centers (Western Electric standard), or the same as for racks ( spacing). Open Rack Open Rack is a mounting system designed by Facebook's Open Compute Project that has the same outside dimensions as typical 19-inch racks (e.g. 600 mm width), but supports wider equipment modules of . Gallery Pictures of 19 inch racks See also Data center Horizontal pitch Rack unit Transit case, for rack cases Open Compute Project, wider, deeper rack design References External links Computer enclosure Mechanical standards Server hardware American inventions 1922 introductions