A tile is a thin slab of terra cotta or other kinds of pottery and porcelain, used in building to cover surfaces. Roof tiles are commonly unglazed and functional; they are either flat, hooked over roof battens, or curved (often S-shaped) and cemented. Floor and structural tiles are hard and vitreous. Wall tiles, used from ancient times, are often decorated with bas-relief molding, painting, and glazing. Seventeenth-century Delft tiles are famous. Plain glazed wall tiles are now commonly used in bathrooms, etc. By analogy, squares of linoleum, vinyl polymers, and cork are also called tiles.