Before we get into the guitar side of things, it’s important to understand exactly what chorus is. The effect of chorus is heard in music when an instrument or voice plays a note, and another note accompanies it with (almost) the same pitch and time.
Due to the nature of voices and instruments the note won’t be exactly the same pitch, therefore it creates a rich, wide sound which we call chorus. The effect can be heard naturally when listening to a choir or string arrangement with multiple voices, and has since been replicated in a variety of ways with guitar pedals and audio plugins.
In terms of effect history, the first examples of purposely out-of-pitch effects can be found with the Hammond organ in the 1930’s, but later similar sounds were created in 1966 when The Beatles and engineer Ken Townsend created the ADT technique (Automatic double tracking – using tape delay to create a delayed copy of an audio signal which is then combined with the original).
Following that a variety of different effects started appearing in music, however the Boss CE-1 is considered the first commercially available chorus pedal. Released in 1976, it featured a circuit that was modelled after the Roland Jazz Chorus amplifier which was brought out a year earlier.
Although effects such as Tremolo and Vibrato were found in Marshall amplifiers in the sixties, it’s only more recently that more modulation effects, such as phase shifter and ring modulator, have become available.