The PURPLL is a phase-locked-loop (PLL) type pedal. It can be used as a fuzzed-out harmonizer, drone synth and more. Hands down the best and most fully-featured PLL on the market. Worth every penny for just the square wave fuzz tone by itself; the wild harmonies, musical dissonance, arpeggiation and filter sweeps are icing on the cake. It's also a drone synth!