A compound pulley, sometimes referred to as a block-and-tackle system, is a mechanical arrangement of pulleys that permits a trade-off of applied force for distance, according to HowStuffWorks. By slinging rope over and through a system of wheels, a compound pulley permits weights to be lifted and pulled with the application of relatively little force, saving energy and reducing the power needed to accomplish work.
A compound pulley is distinguished from a simple pulley in that the simple pulley is only able to redirect the force needed to lift an object, whereas a block-and-tackle rig is able to cut the force needed for the job in half. If the task at hand involves lifting a 100-pound weight, for example, a rope slung over a simple pulley can be used to redirect the 100 pounds of force needed from an inconvenient upward direction to an easier lateral or downward-pointing direction. A compound pulley, however, requires that a greater length of rope be threaded through at least two pulley wheels and reduces the lifting force needed to 50 pounds. According to HowStuffWorks, still greater reductions are possible as the compound system becomes more elaborate, but the required lengths of rope increase accordingly.