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Hammond Chord Organs generate their tones using electronic tubes (a.k.a. "valves") and coils. These organs have 96 buttons on the left for playing chords with one finger, a three-octave manual on the right, and two bass pedals for playing the tonic and fifth of the currently buttoned chord.
There are tone tablets to modify the timbre of the sound and knobs to adjust the relative volume of the bass, chords, and melody (top key being played).
A couple of relatively odd performance inputs are the rhythm bar, which allows the player to give rhythmic emphasis to chords with the bottom of the left hand, and the expression bar, which allows the player to change the overall volume with the right knee.
An excellent resource is 
The Hammond Chord Organ web page.
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