unit (U)
The unit of height (U), Unit (U), is a unit of measurement for the vertical height of subracks and other slide-in units in 19" distribution frames or racks that meet ANSI/ EIA-310-D-1992 guidelines.
Since the units of measurement for racks are based on inches, the rack height unit, referred to as Unit (U) in the US, has been set at 1.75". This corresponds to 44.45 mm in the metric system and represents one height unit (1 U).
Subracks, cassettes and PCBs are available in all possible height units, from 1 U up to 10 U and more. However, height units with a multiple of 3 U, which corresponds to 133.35 mm, are common.