Tack (square sail)

In sailing, the term "tack" applied to a square sail refers to the lower corner of the sail. Specifically, it is the windward lower corner, the corner on the side of the sail from which the wind is blowing.

When sailing upwind with a square sail, the tack is secured to the hull or a spar, holding that corner down and forward. This allows the sail to catch the wind more effectively and generate forward propulsion. The tack is typically secured by a line also called a tack.

The tack's position is crucial for controlling the shape and efficiency of the square sail. Properly adjusting the tack line can significantly affect the sail's performance, especially when close-hauled (sailing as close to the wind as possible).

Unlike fore-and-aft sails where "tacking" describes a maneuver, for a square sail the term "tack" refers to the physical corner and its associated rigging. The equivalent maneuver for a square-rigged ship is generally referred to as "wearing" or "gybing," not tacking.