Let the Choir Sing, History of Boss Chorus Pedals
Chances are that if you’ve listened to any form of music in the last 40 years (especially from the 1980’s) then you’re familiar with the shimmering sound that goes by the name of chorus.
Technically speaking, the chorus effect is created by duplicating the input signal, modulating the pitch of the duplicate, and combining the two signals again. Chorus pedals and rack units are frequently used by guitarists to produce a thick and warm clean tone and keyboard/organ players to add some pitch modulation.