Generically, a synthesizer capable of playing two notes at a time. However, the term usually refers to certain analog synthesizer keyboards produced in the 1970s which were capable of outputting two control voltages at a time, one corresponding to the lowest note being held on the keyboard, and another corresponding to the highest. (If only one note is held, the two control voltages are the same.) These keyboard were used with synthesizers containing two VCOs, but generally the outputs of the two VCOs were mixed and ran through a single filter and other voice circuitry; the synth did not actually have two complete voices, and the keyboard produced only one gate signal. Most performers found this to be a poor substitute for true polyphony, and this design technique disappeared with the advent of affordable true polyphonic synths in the late 1970s.