Effects units are electronic devices that alter how a musical instrument or other audio source sounds. Some effects subtly ""color"" a sound, while others transform it dramatically. Effects are used during live performances or in the studio, typically with electric guitar, keyboard and bass. While most frequently used with electric or electronic instruments, effects can also be used with acoustic instruments, drums and vocals. Examples of common effects units include wah-wah pedals, fuzzboxes and reverb units.Effects are housed in amplifiers, table top units, ""stompboxes"" and ""rackmounts"", or they are built into the instruments themselves. A stompbox (or ""pedal"") is a small metal or plastic box placed on the floor in front of the musician and connected to his or her instrument. The box is typically controlled by one or more foot-pedal on-off switches and contains only one or two effects. A rackmount is mounted on a standard 19-inch equipment rack and usually contains several different types of effects.While there is currently no firm consensus on how to categorize effects, the following are seven common classifications: distortion, dynamics, filter, modulation, pitch/frequency, time-based and feedback/sustain. Guitarists derive their signature sound or ""tone"" from their choice of instrument, pickups, effects units, and guitar amp.