A Rack rail, or rack strip, is used to mount rackable electronic hardware and 19-inch rack mount accessories within a 19-inch rack. Within a rack a minimum of two rack rails are required to mount equipment. The height of rack rail is determined by the number of rack units required for mounting the equipment. The design of racks and rack rails is specified in ECIA - EIA/ECA-310 Each rack unit (U) is equivalent to 1+3⁄4 in (44 mm). Most rack rail is in sizes from 2 units high (3+1⁄2 in or 89 mm) to 54 units high (78+3⁄4 in or 2,000 mm).