English: Drawing of the movement of an antique pocketwatch from the 16th century. The view is of the back of the watch movement with the cover open. This watch incorporates a stackfreed, a rare mechanism that was used in the 15th and 16th century to even out the force of the mainspring, to improve accuracy. The stackfreed is the black cam (top) with a stiff curving spring with a roller pressing against it (top right). The stackfreed is turned by a pinion gear on the mainspring shaft (right of cam) projecting through the back plate of the watch. The declining retarding force of the spring on the cam as it turned allowed more of the mainspring's force through, compensating for the declining force of the mainspring as it unwound. The watch is wound up by inserting a key on the squared-off shaft of the pinion gear and turning. The numbered wheel (right) adjusted the rate of the watch by adjusting the force of the mainspring. A key was inserted on the squared-off shaft and turned.