This diagram shows an upright action. To function properly each of its many components have to operate together as a unit.
This diagram shows an example of an upright piano action for a taller piano. For almost any piano, the Keyboard is - within a few cms - a given height from the floor. The hammer's stricking-point for many older, taller pianos, can be 60 cms or so above the upper surface of the keyboard. Fitting a mechanism which is of a standard size to such a piano requires a sort of leg, to link the key to the action. This is the 'Sticker' labeled in the diagram.