Telephone with letters on its rotary dial (1950s, UK)

These keys were added to provide signals for anticipated data entry purposes in business applications, but found use in Custom Calling Services (CLASS) features installed in electronic switching systems. Initially, between 19, the keypads for civilian subscriber service omitting the lower left and lower right keys that commonly are assigned to the star (✻) and number sign (#) signals, respectively. For military applications, a fourth, right-most column of keys was added for priority signaling in the Autovon system in the 1960s.

The contemporary keypad is laid out in a rectangular array of twelve push buttons arranged as four rows and three columns of keys.

The development of the modern telephone keypad is attributed to research in the 1950s by Richard Deininger under the directorship of John Karlin at the Human Factors Engineering Department of Bell Labs. Because of the installed abundance of rotary dial equipment well into the 1990s, many telephone keypads were also designed to produce loop-disconnect pulses electronically, and some could be optionally switched to produce either DTMF or pulses.

It was standardized when the dual-tone multi-frequency signaling (DTMF) system was developed in the Bell System in the United States in the 1960s that replaced rotary dialing originally developed in electromechanical switching systems. A telephone keypad using the ITU E.161 standard.Ī telephone keypad is a keypad installed on a push-button telephone or similar telecommunication device for dialing a telephone number.

Dtmf dialing