Please note that the content of this book primarily consists of articles available from Wikipedia or other free sources online. Understanding the forces on sails is important for the design and operation of the sails and whatever they are moving, sailboats, ice boats, sailboards, land sailing vehicles or windmill sail rotors. When air moves past a sail, aerodynamic forces develop. These forces occur along the entire surface of the sails, but can be summed into one net force vector. Net aerodynamic force may be decomposed with respect to a boat's course over water into components acting in six degrees of freedom. Two components with respect to wind direction can also be resolved: drag, which is the component directed down wind, and lift, which is the component normal to the wind and perpendicular to drag. This analysis is important to boat design, operation, balance, stability, seakindliness and seaworthiness.