Most wood flooring is made of hardwoods, such as oak, maple, pecan, beech, and birch. There is solid wood flooring, and laminated flooring, which combines wood layered in different directions for strength and to inhibit warping. Each type of wood is graded considering thickness, characteristics of the wood (defects and knots), lengths and widths of the individual pieces and color. Most wood flooring today is prefinished at the factory for ease of installation and uniformity of finish. Wood floor cover is either nailed or applied with mastic (adhesive).