Leslie
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A special loudspeaker system for an electric or electronic organ in which a rotating baffle causes the sound from the speaker to be thrown forward and back so that the Doppler effect causes the volume and pitch to vary, simulating the effect of a pipe organ's tremulant.
Developed in 1937 by Donald Leslie specifically to enhance the sound of the Hammond Tonewheel organ, it was rejected by Hammond and produced and marketed as an external add-on by Leslie himself. Despite its use becoming almost de rigeur, Hammond resisted it, changing the connectors on the organs to obstruct its connection and banning Hammond distributors from also selling the Leslie speaker. It was only in 1978 after the death of Laurens Hammond that the Hammond business finally acquired the right to build the Leslie speaker into the cabinet of its organs.