A square sail set on the lowest yard of a mast on a sailing ship, particularly on the mizzenmast (the rear mast), used to help control the ship's motion and balance.
From 'cross' (the horizontal yard or spar on which the sail hangs) and 'jack' (a general nautical term for a sail or rigging component). This term dates back to the age of sail in the 17th-19th centuries.
The crossjack might sound small and obscure, but it was crucial for ship handling—controlling the lower sails on the mizzenmast helped sailors balance the ship's movement and keep it stable in storms!
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