Traditionally picture frames have been made of wood, and it remains very popular because wood frames can provide strength, be shaped in a broad range of profiles, and allow innumerable surface treatments. Other materials include metals, e. g. silver, bronze, aluminum, and stiff plastics such as polystyrene. A frame surface may be of any color or texture. Both genuine gilding and imitation gold remain popular, although innumerable other surfaces are to be found in most framing establishments. Some picture frames have elaborate molding, which may refer to the subject matter. Intricate decorations are often made of molded, then gilded plaster over a wood base. Picture frame mouldings come in a wide variety of profiles, generally in some sort of L shape with an upward "lip" and a horizontal rabbet. The rabbet functions as a shelf to hold the frame glazing (if any is to be used), some sort of spacer[disambiguation needed] or mat/matte to keep the object safely behind the inner surface of the glazing, the object itself, and backing boards to protect the object from physical damage and environmental pollutants. The lip extends a proportionate distance up from the edge of the rabbet. It restrains materials in the frame and can be used to help set off or reveal the picture aesthetically.