Touch tone is a type of telecommunication system which utilizes dual tone multi-frequency (DTMF) sounds to relay a message, usually a phone number, to a system. Each number is assigned a tone frequency and therefore can be used to relay a call to a certain destination. It quickly replaced earlier forms of dialing as a more efficient way to place a call.
In the 1970s, a television for a local telephone company compared touch tone dialing to rotary dialing. In the end, it declared the touch tone to be the "greatest thing since the wheel." While the advertisement may have been a play on words, it had a double meaning. Before the invention of touch tone, a rotary wheel was the most common way to dial a phone number. Thus, it is the reason why touching numbers to place a call is termed "dialing."
The rotary dial works by interrupting a steady signal. The number desired was indicated by the number of times the signal was interrupted. The technology, while reliable, was somewhat cumbersome and slow. Also, rotary phones were often heavy and hard to place in too many different locations.
The touch tone was tagged as the replacement technology and had been in development, in some form or fashion, since the 1950s. However, it took a number of years to catch on with the general public as existing phone systems had to be upgraded for the change. Since that time, it has remained the most common dialing method.
The technology has since come to be used in a number of different applications. Touch tone allows users to access computer systems with just the touch of a button. The standard has also allowed for integrated voice response systems so that it is possible for companies to communicate with clients and potential clients without having to go through the expense of having a live individual employed, at least during the initial phases of the call.
While it may be easy to overlook, touch tone phones were always meant for this secondary use of communication with computers. Developers sought the advice of many other companies to see what was needed for this purpose. This led to the development of the star (*) and pound (#) keys. It should be noted that original touch tone phones also had letter keys for A, B, C, and D, but these were later dropped. While those keys still serve some administrative functions, most common phones do not have them.