# 941 Hz 1477 Hz In-Band and Out-of-Band Signaling The list below shows the dual tones used to represent each standard telephone key.
It is standardized under ITU-T Recommendation Q.23. One tone is high frequency while the other is low frequency. With DTMF, each key generates two simultaneous tones of specific frequencies, designed so that a human voice can’t replicate the tones. ‘Touch-Tone’ remained a registered trademark of Bell Systems, and later AT&T, until 1984. Over the following decades, it gradually replaced the earlier Pulse Dialing or Rotary Dialing technology. DTMF was invented by the Bell Telephone Company in 1963. Dual Tone Multi-Frequency is a signal tone generated when buttons are pushed on a telephone keypad – hence the term ‘Touchtone Phone’.
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