A sail is means for redirecting the power of the wind to propel a craft on water, ice or land. In doing so, sails mobilize lifting properties as air passes along the surface and they mobilize drag properties to the degree that air is directed at the surface. When both lift and drag are present, they function similarly to a wing in a vertical orientation. In most cases sails are supported by a mast rigidly attached to the sailing craft, however some craft employ a flexible mount for a mast. Sails also employ spars and battens to determine shape in the axis perpendicular to the mast. As a result, sails come in a variety of shapes that include both triangular and quadrilateral configurations, usually with curved edges that promote curvature of the sail.