What Is a Combination?

$$\text{WTF: What's the Frequency}$$

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The sound made when you press a key on a telephone's dialing pad is made up of two pure frequencies, played at the same time. Each row and each column has its own frequency with each button being composed of the intersection of those two frequencies. For example, when you press "2", you hear 697 Hz and 1336 Hz, like two horns each playing a different note at the same time. Our phones use a total of 7 different frequencies, taken in different pairs, to represent the digits 0-9.