Duophony

A duophonic synthesizer is a synthesizer that has two oscillators. It may therefore play two different notes simultaneously and independently. This is in contrast to a monophonic synthesizer, that can only play one note at a time, and a polyphonic synthesizer, which may produce many notes simultaneously.

Duophonic synthesizers are outdated and are therefore mainly of historical interest. However, some of the duophonic synthesizers that have been manufactured in the past are sought after for their unique sound.

Explanation
When multiple keys are pressed on a typical monophonic synthesizer, it will play the highest, the lowest, or the last note keyed.

A duophonic synth has two independent oscillators and may therefore play two notes simultaneously. Often, one of the oscillators will play the highest note and one will play the lowest. When two notes are played, both sound separately.

When only one note is played, both oscillators may be used to create a more complex sound.