The width and height of rack mountable equipment has been standardized in our audio industry. The rack space is at the core of this standard. A rack space is defined as 1.75 inches from top to bottom. Standard rack equipment conforms to this by either being 1.75 inches high, or some multiple of that. Equipment height is often stated in “rack units,” where 1U (one unit) is 1.75 inches, 2U is 3.5 inches, and so on. When racks are built for equipment the “rack rails” have holes in them that correspond to this spacing so that equipment of any size (as long as it’s a multiple of 1.75 inches) can be mounted.