Two vertical supporting rails allow for the mounting of units. The horizantal space between these mounting rails is set at a minimum of 17.75 inches, so any unit being mounted must necessarily be less than this to allow it to fit into the rack. The maximum allowable width of any piece of mounted equipment (measured edge-to-edge across the so-called "mounting ears"), is the stated 19 inches.
The vertically 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 panels must be compliant multiples of that 1.75 inch 1U unit size.
The panel mounting notches, or holes, which pass the (originally 10-32) mounting screws are standardised for the various panel heights. Vertical symmetry is maintained to allow "right-side-up" or "upside-down" installation.