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A compound bow is a modern bow that uses a levering system, usually of cables and pulleys, to bend the limbs.
The pulley/cam system grants the wielder a mechanical advantage, and so the limbs of a compound bow are much stiffer than those of a recurve bow or longbow.

This rigidity makes the compound bow more energy-efficient than other bows, as less energy is dissipated in limb movement. The higher-rigidity, higher-technology construction also improves accuracy by reducing the bow's sensitivity to changes in temperature and humidity.
The pulley/cam system also confers a benefit called "let-off". As the string is drawn back, the pulleys rotate. The pulleys are eccentric rather than round, and so their effective radius changes as they rotate. By the time the bow is at full draw, the change in pulley radius has approximately doubled the wielder's mechanical advantage, and so less force is needed to hold at full draw. This "let-off" gives compound bows their characteristic draw-force curve: a quick rise to peak force and then diminishing to a much lower holding force. The exact shape of the curve is a function of the pulley geometry, which is a matter of buyer preference.
The compound bow was first developed in 1966 by Holless Wilbur Allen in Missouri, and a US patent was granted in 1969. The compound bow has become increasingly popular. In the United States, the compound is the dominant form of bow.

http://en.wikipedia.org/wiki/Compound_bow