By 1971 the North American network had been prepared for operation with central office codes that permitted the digits 0 and 1 as the middle digit, i.e. with the number format NXX (less N11), where N=2–9, and X=0–9.Bell Telephone Laboratories Staff, Engineering and Operations in the Bell System, 2nd Edition (1984). p.118 Starting in c. 1974, large American cities (Los Angeles and New York City) began introducing office codes of this format, to keep up with demand while postponing area code splits by a few more years.
