The 19-inch rack format with rack-units of was established as a standard by AT&T around 1922 in order to reduce the space required for repeater and termination equipment in a telephone company central office. The earliest repeaters from 1914 were installed in ad hoc fashion on shelves, in wooden boxes and cabinets. Once serial production started, they were built into custom-made racks, one per repeater. But in light of the rapid growth of the toll network, the engineering department of AT&T undertook a systematic redesign, resulting in a family of modular factory-assembled panels all "designed to mount on vertical supports spaced 19 inches between centers. The height of the different panels will vary, ... but ... in all cases to be a whole multiple of 1 inches".