kind of block (pulley) that can be opened on one side to receive the bight of a rope.

Merriam-Webster Online Dictionary
snatch block (noun)
a block that can be opened on one side to receive the bight of a rope
snatch block (Wikipedia)
Block-and-tackle-in-use.svg

A block and tackle 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 together 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.

Hero of Alexandria described cranes formed from assemblies of pulleys in the first century. Illustrated versions of Hero's "book on raising heavy weights" show early block and tackle systems.

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