Definitions for "Double Purchase"
A rope or cable that passes from a lifting device over a block, to a second block attached to the load, and tied off at the first block. This line is double purchased. This allows twice as much load to be raised for a given effort, but the rope or cable must be pulled twice as far, so the total work done remains the same. (See Compound Arbor)
A counterweight system where the cradle travels half of the distance of the bar.
A type of manual rigging that uses blocks and suspension cables to reduce counterweight arbor travel to half of the batten supported.