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A pulley, also called a block and tackle, is a device that can change the direction of a force (ie, pulling downwards to raise something up), or can make it much easier to move something. You can use a pulley to move something very heavy by pulling a rope or chain through the pulley much further than you would without the pulley, but the load feels much lighter while you do it. Pulleys are used frequently in the rigging of ships.