Exterior shutters were originally constructed for light control, privacy, security and protection from the elements. Functional shutters hinge on each side of a window or at the top and swing closed when necessary and can also be mounted on tracks applied to the face of the building. Fixed and operable louvers shed rain, allow air transfer, filter direct sunlight, protect from small flying debris, and provide privacy. Operable louvered shutters allow for greater control of light and ventilation. Solid panels and board-and-batten shutters are generally used for security or protection during severe weather. Non-functional shutters are merely used to enhance the appearance of a building. Functional shutters have traditionally been constructed from solid woods that have proven to endure outside elements but have also been mass-produced in woods less suitable to the outdoors like pine. Finishing wood shutters is recommended for extended life. Composite shutters offer advantages over wood shutters such as resistance to twisting, splitting, and rotting. However, PVC shutters are often hinged with three or more hinges and painted with vinyl-safe paint to keep from bowing in the sun and can require additional hardware. Fiberglass shutters are more dimensionally stable than PVC.