19-inch rack

Lore & Background

The term relay rack appeared in the late 19th century in telephony. By 1911, it was also used in railroad signaling, though early rack dimensions were not standardized.

The 19-inch rack format with rack units of 1.75 inches was established as a standard by AT&T around 1922 to reduce space for repeater and termination equipment in telephone central offices. By 1934, it was an established standard with holes tapped for 12-24 screws with alternating spacings of 1.25 inches and 0.5 inches. The EIA standard was revised in 1992 to comply with public law 100-418, setting the standard U as 44.45 mm.

Reader's Guide

Equipment designed for a rack is described as rack-mount or rack-mountable. The height of mountable equipment is standardized as multiples of 1.75 inches, or one rack unit (U). The industry-standard rack cabinet is 42U tall, though many data centers have taller racks. The 19-inch rack format has remained constant while mounted technology has changed.

Two-post or four-post racks hold most equipment in enterprise data centers and server rooms. For fastening, originally mounting holes were tapped with a particular screw thread. Tapped-hole racks are still used for hardware that rarely changes. The tapped-hole rack was first replaced by clearance-hole racks, which use cage nuts.

The next innovation was the square-hole rack, allowing boltless mounting. For structural support, rack-mountable equipment is traditionally mounted by bolting or clipping its front panel to the rack. Four-post racks have become common, with rear mounting posts. There is no standard for equipment depth or outer rack dimensions, but 4-post racks tend to be 600 mm or 800 mm wide and 600 mm, 800 mm, or 1010 mm deep.