the short-term memory span of many people could not handle seven digits and many dialing errors due to memory lapse might occur (the documentation for these tests is lost). As large cities would have both manual and automatic exchanges for some years, the numbers for manual or automatic exchanges would have the same format (originally MULberry 3456, with three letters then four numbers), which could be either spoken or dialed. In the late 1940s, telephones were redesigned with the numbers and letters displayed on a ring outside the finger wheel to provide better visibility. Push-button pulse dialing Even after the transition to DTMF dialing with push-button keypads in most areas, pulse-dialing telephones continued to be produced for some time, even with keypads for dialing, for use with certain private exchange systems. Some of these can be distinguished visually by the lack of keys with the symbols # and *. Some telephones may have an option to select pulse dialing or DTMF dialing. See also AIOD leads (automatic identified outward dialing) Crossbar switch Dial tone Direct distance dialing (DDD) History of the telephone Single-frequency signaling Stepping switch Title 47 CFR Part 68 References External links "The Invention and Development of the Dial Telphone: The Contribution of Three Lindsborg Inventors" by Emory Lindquist; Spring 1957 issue of The Kansas Historical Quarterly "How Your Dial Phone Works", Popular Science, August 1946—detailed article on subject with illustrations AT&T Archives Director's Cut - Now You Can Dial (YouTube), from the AT&T archives American inventions Telephony signals