Demarest, Charles S. (July 1923). "Telephone Equipment for Long Cable Circuits". Bell System Technical Journal. Vol. 2, no. 3. New York: American Telephone and Telegraph Company. pp. 112–140. Retrieved 9 December 2019.
books.google.com
West, Jill; Dean, Tamara; Andrews, Jean (2015). "Structured Cabling and Networking Elements". Network+ Guide to Networks (Seventh ed.). Boston, MA: Engage Learning. p. 169. ISBN9781305480865. Retrieved 9 December 2019. Racks are measured in rack units (RU or U) with the industry standard being 42U tall — about 6 feet.
Keller, A. C. (January 1964). "Recent Developments in Bell System Relays — Particularly Sealed Contact and Miniature Relays". Bell Labs Technical Journal. 43: 15–44. doi:10.1002/j.1538-7305.1964.tb04057.x.
