In the telephone
industry, a method of signalling over an audio channel. Two or more tones are sounded simultaneously, allowing the communication of information between switching equipment.
The best known multi-frequency signalling is touch-tone. There is also the lesser known MF used between Bell System switches before electronic switching became commonplace. The Blue Box was used by phone phreaks to make free phone calls by generating these tones and fooling the phone system into thinking the phreak is one of its own switches.