Sails are panels of canvas joined together and used to propel a vessel. Each sail functions in a manner similar to a propeller blade. As airflow passes over the sail, a region of reduced pressure forms on its leeward (convex) side, while increased pressure develops on the windward (concave) side.

The resulting pressure difference generates a force that produces thrust, driving the vessel forward, as well as a lateral force that causes drift, pushing the sailing vessel to leeward.

Comprehensive guide to sailing ship classification by sail rigging types: square, fore-and-aft, and combined rigs with comparison table and examples.