In sailing, different sail trims, or adjustments, are necessary in order to move the boat from one point to another. Whenever a sailboat needs to progress in a direction heading into the wind, the operation necessary to keep the bow turning through the wind and alternately filling the sails from one side and then the other, is called tacking. Tacking is required because a sailboat is not capable of sailing directly into the wind. It must effectively zigzag against it in order to make headway.
Tacking begins by being close-hauled on either a starboard or port tack. Close-hauled means that the boat is moving (or tacking) as close into the wind as possible and the sail is trimmed tightly to harness the present wind direction from either the left side or right side. If the wind is blowing from off the left side of the boat to glance against the left side of the sail, the boat is said to be on a port (left) side tack. If the reverse is true, it is said to be on a starboard (right) side tack.
Tacking refers to the maneuver of changing from one side to the other. To accomplish this, it is first important to determine there is enough room to safely change direction. If there is sufficient space for the maneuver, the helmsman (person steering the boat) will call out, “Ready to come about!” When the mate (crewman) is in position and ready to respond, he or she replies, “Ready!” Upon hearing this, the helmsman will turn the bow of the sailboat into the wind through use of the rudder. As the turn progresses, the sail will move away from its current best position of harnessing the wind direction to a weaker position wherein the sail will luff (flap).
At this point two very important things must occur simultaneously. As the helmsman continues to turn the bow (front part of boat) into and through the wind, the mate will loosen the sheet (line connected to sail) to allow the sail to move and fill from the other side. As the bow moves through the wind and the sail fills on the opposite side, the helmsman and crew must shift their position to sit on the windward side of the boat. As the helmsman steadies the bow to continue on its new course, the mate tightens (trims or secures) the sail so it does not luff. This is called "sheeting it home" and completes the tacking maneuver.