A chorus effect duplicates the signal, delays the copy by a very short, constantly changing amount and mixes it back with the original. The moving delay creates small shifts in pitch, so a single instrument starts to sound like several players not quite in unison, as in an ensemble. Roland built a stereo chorus into its JC-120 Jazz Chorus guitar amplifier in 1975, and its Boss brand put the circuit into a pedal, the CE-1 Chorus Ensemble, the following year.
Chorus is commonly applied to electric pianos, guitars and synth pads. In jazz lo-fi it adds shimmer and width to Rhodes chords or clean guitar; pushed further, its slow wobble overlaps with the tape-wow effects that make a part sound pleasantly worn. This entry covers the effect, not the chorus of a song or a jazz solo.