A stackfreed is a simple spring-loaded cam mechanism used in some of the earliest antique spring-driven clocks and watches to even out the force of the mainspring, to improve timekeeping accuracy. Stackfreeds were used in some German clocks and watches from the 16th to the 17th century, before they were replaced in later timepieces by the fusee. The term may have come from a compound of the German words starke ("strong") and feder ("spring"). History Spring-driven clocks were invented around 1400 in Europe. Mainsprings allowed clocks to be portable and smaller than the previous weight-driven clocks, evolving into the first large watches around 1500. However, the early spring-driven timepieces were much less accurate than weight-driven clocks, because the drive force (torque) exerted by a coiled spring, unlike a weight, is not constant, but is maximum when the spring is wound up and declines as the spring unwinds to turn the movement's wheels. The main cause of inaccuracy in early spring-driven timepieces was the large variation in force provided by the mainspring as it unwound during the timepiece's running period. The force of the mainspring, transmitted through the clock's gears, gives pushes to the oscillating balance wheel which keeps time. The primitive verge and foliot movement used in all early timepieces was very sensitive to the amount of force applied to it, particularly before the balance spring was added in 1658; the weaker the drive force applied by the mainspring, the slower the balance wheel would oscillate back and forth. So without some device to equalize the force of the mainspring, early clocks and watches slowed down drastically during the clock's running period as the mainspring lost force, causing inaccurate timekeeping. Two devices appeared in the first spring powered clocks to even out the power of the mainspring: the fusee and the stackfreed. The origin of the stackfreed is unknown. It is assumed it was invented in the southern Germanic states (Nuremberg and Augsburg) during the 16th century, since the early spring clocks which incorporated it came from there, but it may have been invented earlier. Drawings of stackfreeds appear in Leonardo da Vinci's Codex 1 (1492-1497) and M3 (1497-1499); possibly the device was brought to his attention by his German assistant Giulio. While the fusee went on to become the standard mainspring equalizer in European timepieces, the less satisfactory stackfreed was used exclusively in a few German timepieces; and disappeared