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Telephone keypad
A telephone keypad is a keypad that appears on a "Touch Tone" telephone. It was standardised when the dual-tone multi-frequency system in the new push-button telephone was introduced in the 1960s, which gradually replaced the rotary dial.〔(Engineering Pathway – Bell Telephone introduces push button telephone ) – by Alice Agogino – November 18th, 2009〕 The invention of the keypad is attributed to John E. Karlin, an industrial psychologist at Bell Labs. The contemporary keypad is laid out in a 4×3 grid, although the original DTMF system in the new keypad had an additional column for four now-defunct menu selector keys. Most keypads have a ''star key'' or ''asterisk key'' on the bottom left and a ''hash key'' on the bottom right.
==Key frequency==
When used to dial a telephone number, pressing a single key will produce a dual-tone multi-frequency signaling pitch consisting of two simultaneous pure tone sinusoidal frequencies. The row in which the key appears determines the ''low'' frequency, and the column determines the ''high'' frequency. For example, pressing the '1' key will result in a sound composed of both a 697 and a 1209 hertz (Hz) tone.
抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』
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