Telephone switches introduced the idea of frames. They were the first systems to have enough components to require modular organization. Western Electric tended

Telephone switches introduced the idea of frames. They were the first systems to have enough components to require modular organization. Western Electric tended to use 24 inch spacing. 19 inch racks seem to come from railroad interlocking and may have been introduced by Union Switch and Signal, founded by Westinghouse in 1881, and still around, as a unit of Ansaldo.