1. n. [Drilling]

The set of sheaves that move up and down in the derrick. The wire rope threaded through them is threaded (or "reeved") back to the stationary crown blocks located on the top of the derrick. This pulley system gives great mechanical advantage to the action of the wire rope drilling line, enabling heavy loads (drillstring, casing and liners) to be lifted out of or lowered into the wellbore.

See related terms: block,  crown block,  drawworks,  drillstring,  hook,  liner,  sheave,  slip and cut