19-inch rack Overview and history
The 19-inch rack format has remained constant while the technology 
that is mounted within it has changed considerably and the set of fields to
 which racks are applied has greatly expanded. The 19-inch (482.6 mm) standard 
rack arrangement is widely used throughout the telecommunication, computing, audio, video, 
entertainment and other industries, though the Western Electric 23-inch standard, with holes
 on 1-inch (25.4 mm) centers, is still used in legacy ILEC/CLEC facilities.
A 19-inch rack used for switches at the DE-CIX in Frankfurt, Germany
19-inch racks in 2-post or 4-post form hold most equipment in modern data centers,
 ISP facilities and professionally designed corporate server rooms. They allow for 
dense hardware configurations without occupying excessive floorspace or requiring shelving.
19-inch racks are also often used to house professional audio and video equipment, 
including amplifiers, effects units, interfaces, headphone amplifiers, and even small 
scale audio mixers. A third common use for rack-mounted equipment is industrial power, 
control, and automation hardware.
Typically, a piece of equipment being installed has a front panel height 1⁄32 inch 
(0.031 inches or 0.787 millimetres) less than the allotted number of Us. Thus, a 1U rackmount 
computer is not 1.75 inches (44.4 mm) tall but is 1.719 inches (43.7 mm) tall. 2U 
would be 3.469 inches (88.1 mm) instead of 3.5 inches (88.9 mm). This gap allows a bit 
of room above and below an installed piece of equipment so it may be removed without binding
 on the adjacent equipment.
In 1965, a durable fiber reinforced plastic 19-inch rackmount case was patented by ECS 
Composites and became widely used in military and commercial applications for electronic 
deployment and operation. State-of-the-art rackmount cases are now also constructed of thermo 
stamped composite, carbon fiber and DuPont’s Kevlar for demanding military and commercial uses.