A method of dialing a telephone that Bell System developed in the late 1960's. A lot of us are old enough to remember rotary dials, and how long it takes to dial a phone number with them. Touch-tones solved the problem by allowing one to dial by simply pressing buttons.

Touch-tones were an extension on the multi-frequency technology already used within Bell System's telephone switching network. A digit would be represented by two tones sounded simultaneously. The tones were chosen such that it would be unlikely for them to be inadvertantly generated by natural means, such as a person speaking. They also had to be different from the the other tones currently used by the switching network.

Touch-tone changed the way everyone used phones; we still use it today, not just for dialing but also for making choices in voice-mail systems. What's ironic is that even though hardly anyone uses rotary dialing anymore, a lot of telephone companies still charge extra for the ability to use touch-tone.

Here's what a normal touch-tone keypad looks and sounds like:

        1209   1336   1477
         Hz     Hz     Hz

        +---+  +---+  +---+
697 Hz  | 1 |  | 2 |  | 3 |
        |   |  |abc|  |def|
        +---+  +---+  +---+

        +---+  +---+  +---+
770 Hz  | 4 |  | 5 |  | 6 |
        |ghi|  |jkl|  |mno|
        +---+  +---+  +---+

        +---+  +---+  +---+
852 Hz  | 7 |  | 8 |  | 9 |
        |prs|  |tuv|  |wxy|
        +---+  +---+  +---+

        +---+  +---+  +---+
941 Hz  | * |  | 0 |  | # |
        |   |  |   |  |   |
        +---+  +---+  +---+

As explained in the node DTMF (dual-tone multi-frequency), each button produces a sound consisting of two different frequencies, the lower one for the row the button is in, and the higher one for the column. A fourth column of buttons, labelled A, B, C, D (or FO, F, I, P) from top to bottom, had a column frequency of 1633 Hz; these were used to indicate the priority of a call in the Autovon system, but are absent from many phones.

Part of what makes the combinations of tones unlikely to be generated accidentally is that no two of these frequencies produce the simple ratios corresponding to an octave (2:1), perfect fifth (3:2), perfect fourth (4:3), or major third (5:4).

A list of some tunes you can play on a touch-tone phone can be found at http://www.thisisarecording.com/touchtonetunes/. If you want to play music on your phone, do be sure to call someone first—not only so that you have an audience, but also so that your tinny rendition of "Pop Goes the Weasel" doesn't show up on your long-distance bill.

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