Except when head to wind, a boat is said to be on either a port or starboard tack. If the wind is coming from anywhere on the port side, the boat is on port tack. Likewise if the wind is coming from the starboard side, the boat is on starboard tack. For purposes of the International Regulations for Preventing Collisions at Sea and the Racing Rules of Sailing, the wind is assumed to be coming from the side opposite that which the mainsail boom is carried, if the vessel is so rigged. The direction of sail is used for square-rigged vessels.