The hole spacing for standard 19″ racks on the mounting flange is spaced in groups of three holes. This three hole group is defined as a Rack Unit (RU) or sometimes just call a “U”. 1U occupies 1.75″ (44.45 mm) of vertical space.
Manufacturers of rackmount equipment make their product lines based upon how many RUs that the equipment occupies in the server rack. The three hole group spacing is measured from center hole to center hole and does not vary, regardless of whether the server rack has square or round mounting holes. This uneven hole spacing can cause installation issues if the installer does not pay close attention to where the slide rail assemblies are being installed. The most common problem is that the slide rail assembly hooks or holes do not line up with the holes in the server rack. This is typically due to the slide rail assemblies being misaligned with the hole pattern or otherwise referred to as “being off a U”.