Two vertical supporting rails allows for the mounting of units. The horizontal space between these mounting rails is set at a minimum of 17.75 inches, the vertical placed screw holes in the rack are spaced at 0.50 inch and 0.60 inch (on an alternating basis). This is totally in harmony with the standard 1.75 inch panel height, known as 1U, and ALL installed panel must be compliant multiple of that 1.75 inch 10 unit size.