An effects peal (or a "Stomp Box") is an electronic effects unit housed in a small metal or plastic chassis used by musicians, usually electric guitar players, to modify their instrument sound. Musicians playing electronic keyboards, electromechanical organs, the electric bass, or electric violin also use effects pedals. These devices alter the sound quality or timbre of the input signal, adding effects such as distortion, fuzz, overdrive, chorus, reverberation, wah-wah, flanging, phaser or pitch shifting. The sound of a guitar or other instrument that is played without an effects pedal is described as "clean", "straight" or "dry."