Encoding of numbers as pairs of audio tones to make them easy to transmit over telephony circuits optimized for the frequencies involved in human speech. Given a standard telephone keypad with four rows of three buttons each, each row and column is given a fixed audio frequency. The four rows are assigned frequencies of 697 Hz, 770 Hz, 852 Hz, and 941 Hz. The three columns are assigned frequencies of 1209 Hz, 1336 Hz, and 1477 Hz. So the number 5 is represented by two tones at 770 Hz and 1336 Hz. Some keypads have a fourth column at the right, which is assigned the tone frequency of 1633 Hz.