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 The height of the electronic modules is also standardized as multiples of 175 inches (4445 mm) or one rack unit or U (less commonly RU) The 19inch rack format with rackunits of 175 inches (4445 mm) was established as a standard by AT&T around 1922 in order to reduce the space required for repeater and termination equipment in a telephoneMM6 Enhanced Channel Rack Black 65inch Deep channel 7ft High 45 RU tapped #1224A rack unit (abbreviated as U, less commonly seen as RU) is a unit of measurement applied to equipment racks and the servers, disk drives and other devices that they contain One U is 175 inches (4445mm); 2u 19 Rack Mount Cable Duct Cm2upcs Selby Rack ru numbering