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Block and tackle
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<p>A block and tackle or only tackle is a system of two or more <a href="/facts/Pulley/Mrq15sdg">pulleys</a> with a <a href="/facts/Rope/heTalcCM">rope</a> or <a href="/facts/Wire_rope/9z4b9SYB">cable</a> threaded between them, usually used to lift heavy loads. </p><p>The pulleys are assembled to form <a href="/facts/Block_(sailing)/INE3Cokx">blocks</a> and then blocks are paired so that one is fixed and one moves with the load. The rope is threaded through the pulleys to provide <a href="/facts/Mechanical_advantage/tWX3c82Q">mechanical advantage</a> that amplifies the force applied to the rope. </p><p><a href="/facts/Hero_of_Alexandria/z1fSvztI">Hero of Alexandria</a> described cranes formed from assemblies of pulleys in the first century. Illustrated versions of Hero's <i>Mechanica</i> (a book on raising heavy weights) show early block and tackle systems. </p>