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.