Tack is a term used in sailing that has different meanings in different contexts, variously a part of a sail, and an alignment with the wind. When using the latter sense, the maneuver of turning between starboard and port tack is either tacking or jibing.
The tack is the lower corner of the sail's leading edge. On a sloop rigged sailboat, the mainsail tack is connected to the mast and the boom at the gooseneck. On the same boat, a foresail tack is clipped to the deck and forestay.
Tack is the alignment of a sailing vessel with respect to the wind when moving upwind: that is, when the vessel's bow is pointed within 90 degrees of the wind. If the wind is to starboard, the vessel is on "starboard tack", and if to port, on "port tack".
The "rules of the road" for ships and boats declare that when the courses of two sailing vessels converge, the vessel on port tack must give way to a vessel on starboard tack. For this purpose, port and starboard tack include any position with the wind to that side, whether moving upwind or downwind. If a vessel is fore-and-aft-rigged, the actual wind position is overridden by the position of the boom (the mainsail boom in a vessel with multiple masts), which is assumed to be on the side opposite the wind, even if the vessel is running straight downwind or is in the act of tacking; that is, if the boom is to port the vessel is on starboard tack, and vice versa. There are exceptions to the requirement to give way: in particular, if the vessel on port tack does not have sea-room to tack or maneuver out of the way.
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