Several of my former colleagues have been asking about the venerable 19-inch rack. Why is it 19 inches, and why has that dimension persisted as electronics has shrunk dramatically?
Wikipedia traces the term “relay rack” back to at least 1911, although “there is little evidence that dimensions of that early rack were standardized”—at least until 1934.
Practical-Home-Theater-Guide.com traces the 19-inch rack back to George Westinghouse in 1890, who used 19-inch shelving to house railroad relay gear. The site notes that telephone companies eventually adopted the 19-inch width, and the EIA standardized the 19-inch structure in 1965 with its 310-D standard.
Wikipedia notes that the relevant EIA standard was revised in 1992 to set the standard U at 1.752 inches (44.50 mm), raising another question, why was the rack unit height set at about 1.75 inches?
Wikipedia adds little on that question, simply noting, “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….”
The Server Rack FAQ emphasizes this interesting fact: “Nothing on a 19-inch rack measures 19 inches.” What’s important for a 19-inch rack is the hole-to-hole spacing, which is 18.312 inches, and the minimum opening, which is 17.75 inches.
As to the persistence of the 19-inch dimension throughout the years since 1890, you can today fit a lot in a lot more functionality (think 3U PXI) than Westinghouse could. And if you don’t need the full 19 inches, you can buy a half-rack chassis or instrument.
Nevertheless, the reasons for the original choice of 19 inches seem lost in history. If you have any information please leave a comment.