19-inch rack Rack Unit
Racks are divided into regions, 1.75 inches (44.45 mm) in height, 
within which there are three complete hole pairs in a vertically symmetric pattern, 
the holes being centered 0.25 inches (6.35 mm), 0.875 inches (22.23 mm), and 1.5 inches 
(38.10 mm) from the top or bottom of the region. Such a region is commonly known as a U, 
for "unit", or in German "HE" (for Höheneinheit) and heights within racks are measured by 
this unit. Rack-mountable equipment is usually designed to occupy some integer number of U.
 For example, an oscilloscope might be 4U high, and rack-mountable computers are mostly between
 1U and 4U high. A blade server enclosure might require 10U.