A tack is the windward side of a sailing craft (side from which the wind is coming while under way)--the starboard or port tack. Generally, a craft is on a starboard tack if the wind is coming over the starboard (right) side with sails on port (left) side. Similarly, a craft is on a port tack if the wind is coming over the port (left).

Tack is also a nautical term both for the lower, windward corner of a sail.

Windward side of course sailed

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