Page "Dual-tone multi-frequency signaling" Paragraph 21

The DTMF keypad is laid out in a 4 × 4 matrix, with each row representing a low frequency, and each column representing a high frequency.

Pressing a single key ( such as ' 1 ' ) will send a sinusoidal tone for each of the two frequencies ( 697 and 1209 hertz ( Hz )).

The original keypads had levers inside, so each button activated two contacts.

The multiple tones are the reason for calling the system multifrequency.

These tones are then decoded by the switching center to determine which key was pressed.