1. n. [Drilling]
The fixed set of pulleys (called sheaves) located at the top of the derrick or mast, over which the drilling line is threaded. The companion blocks to these pulleys are the traveling blocks. By using two sets of blocks in this fashion, great mechanical advantage is gained, enabling the use of relatively small drilling line (3/4- to 1 1/2-in-diameter steel cable) to hoist loads many times heavier than the cable could support as a single strand.
See: block, drawworks, mast, sheave, slip and cut, traveling block