Phasers produce sounds similar to flange and chorus effects, but the phasing and replication of the original sound signal produce a "spacier" effect than the more natural tone of these other effects. The term phasing or phaser was originally used when referring to the flange effects of the 1960s.
In the late 1960s phasers became available in the form of guitar pedals. The Univox Uni-Vibe phase shifter was introduced to the North American market in 1968 and was added to the gear used by guitarists like Jimi Hendrix. The Uni-Vibe, also known as the Jax Vibra-Chorus, was originally released in Japan by the Shin-ei company with the intention to reproduce similar sounds of a Leslie speaker.
Another one of the earliest phasers, the MXR Phase 90, was released in 1972 and helped launch the MXR brand in the following years. The MXR Phase 90 is shaped differently than a lot of bulkier effects pedals of its time, with a slim rectangular enclosure and just one control knob to adjust the phasing speed. In the early 1980s Jim Dunlop bought the MXR brand after they went bankrupt and started the production of the new version of the MXR Phase 90 adding more features to the original pedal.