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"The main valve controls admission, release, and compression; the other (valve) regulates the cut—off only, which may be changed without in any way affecting the other events of the stroke. This cut—off valve may be placed in a separate valve chest, or it may be placed in the back of the main valve." —Derr, 1911

Close up View of Double Sliding Valve Gears from Steam Engine

"The main valve controls admission, release, and compression; the other (valve) regulates the cut—off…