A chorus pedal can add incredible depth and richness to your sound. You’re likely familiar with the term “chorus” in the context of singing where multiple singers sing in unison. A guitar chorus effect pedal does virtually the same thing — it makes your guitar sound like two or more guitars playing in unison. It does this by duplicating the notes you are playing, slightly adjusting the timbre and pitch, then converging the new sound with your original sound to add depth.
What makes chorus work is that it really does try to act like two or more players playing at the same time. In real life, two people playing the exact same thing will never be completely synchronized. They’ll start each note at a slightly different time, perhaps pitch the notes slightly differently, hold them differently, etc. They can try to mimic or mirror each other, but the mimicry will never be exact. This slight “offness” is what makes a chorus sound different from a single voice — the slight variances in the signals make the sound richer and fuller.
Essentially, a guitar chorus pedal works by splitting the guitar signal into two pieces, then delaying one of those pieces a fractional amount. The delay is very slight, however. Too much would yield a reverb effect. Instead, the delay is just enough to shift the wave form of one signal path so that it no longer exactly matches the original signal path.
For example, if you visualize a wave form as a sine wave on a graph. Now, take the exact same wave form, shift it just a fraction, and place it on the same graph as the original wave form. The result is that when one form peaks, the other will be slightly behind. The effect is that th tone will sound much like two guitars playing in unison. It creates a fuller, richer, “shimmering” sound to your piece.
Significantly, a chorus effect doesn’t really alter the pitch of the tone — it doesn’t try to sound like two guitars playing in harmony (octaves, fifths, fourths, etc). Again, while the chorus effect is slightly similar to a digital delay or reverb effect, it’s very slight — just enough to give the sound that typical chorus “shimmer.”
Most choral effects only add a second guitar signal to the mix, although some pedals have the ability to add three or more guitars (think ZZ Top). Signficantly, a chorus is effective on a clean guitar signal as well as on a distorted one. A clean chorus will also fill a room better than a straight unaltered signal. The effect is more pronounced when notes are sustained for longer periods.
For tips and tricks on when to use a chorus pedal, check out How to Effectively Use a Chorus Pedal.