The Leslie speaker is a combined amplifier and two-way loudspeaker that projects the signal from an electric or electronic instrument, while modifying the sound by rotating the loudspeakers. It is most commonly associated with the Hammond organ, though it was later used for the guitar and other instruments. A typical Leslie speaker contains an amplifier, and a treble and bass speaker—though specific components depend upon the model. A musician controls the Leslie speaker by either an external switch or foot pedal that alternates between a slow and fast speed setting, known as "chorale" and "tremolo".