Tracker bar

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The bar over which the paper piano roll passes in a player piano, and in many self-playing organs and orchestrions. It has a hole for each note, connected by a narrow pipe to its corresponding pouch and valve in the stack or valve chest. Air is drawn in when a hole in the paper coincides with a hole in the tracker bar, lifting the pouch in the valve chest, which opens the bigger valve opposite it allowing the suction inside the valve chest to be applied to the pneumatic which collapses and operates the piano action for the relevant note. The tracker bar is usually made of brass.