Tremolo produces a periodic variation in the amplitude (volume) of the note or chord, which creates a "shuddering" effect. A sine wave applied as input to a voltage-controlled amplifier produces this effect. Tremolo effects normally have a "rate" knob which allows a performer to change the speed of the variation. Some tremolo effects also have a "depth" knob. The "tremolo" effect described here should not be confused with the mis-named "tremolo bar", a device on a guitar bridge which allows the player to create a vibrato or pitch-bending effect.