A block and tackle is a system of two or more pulleys with a rope or cable threaded between them, usually used to lift or pull heavy loads.
The most common arrangement is to have a set of fixed pulleys or "sheaves" all mounted on a single axle, and another set left to move. Each set is called a block (the whole assembly, with the rope, is the "tackle"). The mechanical advantage of a block and tackle is equal to the number of lines running between the two blocks. For example, a block and tackle consisting of three fixed and three moving pulleys has six lines going between the pulleys, which gives it a mechanical advantage of 6. This enables the lifting of weights six times heavier than would be possible otherwise.
The increased force produced by a block and tackle is offset by the increased length of rope needed. In order to raise a block and tackle with a mechanical advantage of 6 a distance of 1 metre, it is necessary to pull 6 metres of rope through the blocks.
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