Harmonizer
An effects unit that shifts a signal's pitch in real time and lays it alongside the original as a harmony.
The name is Eventide's trademark, from the H910 of 1975, one of the first digital boxes able to change pitch without changing tempo. The method was crude: audio was written into memory and read out at a different rate, and where the read and write pointers collided the splices were crossfaded. It glitched, and the glitches became part of the sound. Tony Visconti ran the snare through one on David Bowie's Low in 1977.
Later generations learned to follow a key, so the harmony lands on a third or fifth that actually fits the chord rather than a fixed interval. Phase vocoders and time-stretching have made the technique cleaner, but that characteristic splice survives as a preset of its own.