Two Blocks: The system includes two blocks—one fixed and one movable—each typically containing two sheaves.
Multiple Lines: A line is reeved through the sheaves in a way that creates four parts of line supporting the load.
Mechanical Advantage: This setup provides a 4:1 lifting advantage, meaning the force required to lift a load is reduced to roughly one-quarter of its weight (not accounting for friction).
Applications:
Hoisting sails, spars, or cargo
Adjusting rigging tension
Lowering boats or gear over the side
This configuration was common on traditional sailing vessels and is still used today in various forms for lifting tasks where controlled, distributed force is needed.
