Forces on sails apart, which generally prevents heeling. Each sailing craft is a system that mobilizes wind force through its sails—supported by spars and rigging—which provide motive power and reactive force from the underbody of a sailboat—including the keel, centerboard, rudder or other underwater foils—or the running gear of an ice boat or land craft, which allows it to be kept on a course. Without the ability to mobilize reactive forces in directions different from the wind direction, a craft would simply be adrift before the wind. Accordingly, motive and heeling forces on sailing craft are either "components of" or "reactions to" the
Forces on sails mainsail causes the stagnation line on the jib to be shifted forward and allows the boat to point closer to the wind, owing to higher leeward velocities of the air over both sails. Sails characteristically have a coefficient of lift ("C") and coefficient of drag ("C") for each apparent wind angle. The planform, curvature and area of a given sail are dominant determinants of each coefficient. Sails are classified as "triangular sails", "quadrilateral fore-and-aft sails" (gaff-rigged, etc.), and "square sails". The top of a triangular sail, the "head", is raised by a halyard, The forward lower corner of the sail,