The "Touch Tone" telephone was standardized when the dual-tone multi-frequency (DTMF)
system in the new push-button telephone was introduced in the 1960s, and replaced the rotary dial. When used to dial a telephone number, pressing a single key will produce a pitch consisting of two simultaneous pure tone sinusoidal frequencies. The row in which the key appears determines the low frequency, and the column determines the high frequency - hence the name dual tone mutli-frequency. Canon 40D, 1/20 sec @ f/2.8, ISO 800, 50mm macro.