Originally Posted by: LordathestringsThe advent of digital processing made some amazing things possible. Like changing the pitch of a note fed into a harmonizer into another note a fixed interval away from the original. The new note is then mixed with the original, and sent out as a single signal.

That's a bit over simplified, but it catches the essence.

That's pretty much it and it's selectable. You choose a key and the interval off the key to harmonize.