The sides of a shade sail are curved to ensure proper tensioning. The amount of the curve (catenary curve) depends on the shape of the shade sail and can be adjusted when ordering a custom shade sail. Curvature is not solely for tensioning purposes, it can be increased (to go around a tree or other obstruction) or decreased in some cases. Our design software automatically optimizes the catenary curve for each design. Please get in contact with our design team for any unique requirements.