A block and tackle[1][2] or only tackle [3] is a system of two or more pulleys with a rope or cable threaded between them, usually used to lift heavy loads.

The pulleys are assembled to form blocks 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 mechanical advantage that amplifies the force applied to the rope.[4]

Hero of Alexandria described cranes formed from assemblies of pulleys in the first century. Illustrated versions of Hero's Mechanica (a book on raising heavy weights) show early block and tackle systems.[5]

Typical values are 1.04 for roller bearing sheaves and 1.09 for plain bearing sheaves (with wire rope).[11]

The increased force produced by a tackle is offset by both the increased length of rope needed and the friction in the system. 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. Frictional losses also mean there is a practical point at which the benefit of adding a further sheave is offset by the incremental increase in friction which would require additional force to be applied in order to lift the load. Too much friction may result in the tackle not allowing the load to be released easily,[notes 1] or by the reduction in force needed to move the load being judged insufficient because undue friction has to be overcome as well.

When installing a block on an existing line, it is often inconvenient at best to thread the rope through the block to be added.