Harmonium Basic Model Mechanism Schematic. 

Three Styles of Key Construction 

The Solid Key:  The Key top and damper pads are mounted on a single shaft, and there is a vertical center pin hinge and a single guide pin. This style is cheap and simple with the main drawback being that the bass and treble vents don't open evenly.

The Stick and Pallet:  With this style, the pallet lift is parallel to the sound board, and allows for a balanced air flow. 

The Stick, Lever, and Pallet: This style is  only found on expensive, 3 bank, coupler, scale/changer model harmoniums. There are more moving  parts involved, including the Coupler Sticks which run diagonally underneath the keys, and the lever pins.