The is a set of sheaves (pulleys) that move up and down in the derrick. The drilling line is threaded (reeved) over the sheaves on the crown and through the sheaves in the traveling block. This provides a great mechanical advantage to the drilling line, enabling it to lift heavy loads of pipe and casing. The number of the pulleys used on the two Blocks can vary from 5 to 8, providing a variable capacity for the Hoisting system.