Tack (sailing)

A tack is a nautical term both for the lower, windward corner of a sail and, "separately", for the side of a sailing craft from which the wind is coming while under way—the starboard or port tack. A boat is on a starboard tack if the wind is coming over the starboard (right) side of boat with sails on port (left) side. Similarly, a boat is on a port tack if the wind is coming over the port (left) side of boat.
Sail corner.
The tack is the corner on a fore-and-aft sail where the "luff" (the forward edge) and "foot" (the bottom edge) connect and, on a mainsail, is located near where the boom and mast connect. On a square sail or a spinnaker, the tack is the windward "clew" (lower corner) and also the line holding down that corner; when the vessel changes course to have the other vertical edge of the sail to the wind, the other clew becomes the tack.
Wind direction on sailing vessel.
As a point of reference, tack is the alignment of the wind with respect to a sailing craft under way. If the wind is from starboard side of the sailing craft, it is on "starboard tack", and if from port, on "port tack". The International Regulations for Preventing Collisions at Sea for vessels underway declare that when the courses of two sailing vessels converge, the vessel on "port tack" must give way to a vessel on "starboard tack". The maneuver of changing a sailing craft's course from one tack to the other with the wind coming across the bow is called tacking or with the wind coming across the stern is called jibing for fore-and-aft rigged sailing craft or "wearing ship" for square-rigged vessels.