The height unit (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-1992guidelines
. Since the units of measurement for racks are based on inches, the rack height unit, referred to as Unit (U) in the U.S., has been set at 1.75". This is equivalent to 44.45 mm in the metric system
and represents one height unit (1 U).printed circuit boards 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.