Looking further-
http://www.answers.com/topic/plywoodPlywood may be made from hardwoods, softwoods, or a combination of the two. Some common hardwoods include ash, maple, mahogany, oak, and teak. The most common softwood used to make plywood in the United States is Douglas fir, although several varieties of pine, cedar, spruce, and redwood are also used.
There are two broad classes of plywood, each with its own grading system.
One class is known as construction and industrial. Plywoods in this class are used primarily for their strength and are rated by their exposure capability and the grade of veneer used on the face and back. Exposure capability may be interior or exterior, depending on the type of glue. Veneer grades may be N, A, B, C, or D. N grade has very few surface defects, while D grade may have numerous knots and splits. For example, plywood used for subflooring in a house is rated "Interior C-D". This means it has a C face with a D back, and the glue is suitable for use in protected locations. The inner plies of all construction and industrial plywood are made from grade C or D veneer, no matter what the rating.
The American Plywood Association identifies several grades of product. Plywood is designated group 1 when made from northern-grown Douglas-fir, western larch, and such southern pines as loblolly and longleaf, or other woods noted for their strength. Plywoods in groups 2, 3, and 4 are made from woods of successively lower strengths. Consequently, group 1 plywood offers the greatest stiffness, group 4 the least.
