US Patent 7241179 Universal audio jack and plug

A universal four-contact plug and jack assembly permits interconnection of microphone and stereo audio signals between an audio peripheral incorporating the four-contact plug and an audio device that includes the four-contact jack. Dimensionally, the interconnecting segments of the four-contact plug are, in terms of overall length, consistent with standard three-contact audio plugs which allows the four-contact plug to be plugged into a three-contact jack intended for use with conventional three-contact audio plugs. Similarly, the four-contact jack has three of its contacts positioned as a conventional three-contact jack but with a fourth contact position to interconnect with the additional contact of four-contact plugs. With this configuration, an audio device including the four-contact plug is compatible with audio peripherals including stereo headsets, monaural headsets with microphones, and others.