To minimize the anisotropic properties of natural wood and ensure uniform properties and stable shape, plywood generally adheres to two basic structural principles: symmetry; and perpendicularity of the fibers in adjacent layers. The symmetry principle requires that the veneers on either side of the plywood's central plane of symmetry be symmetrical, regardless of wood properties, veneer thickness, number of layers, fiber orientation, moisture content, and other factors. Within a single sheet of plywood, veneers of a single species and thickness can be used, as can veneers of different species and thicknesses. However, any two symmetrical layers of veneer on either side of the central plane of symmetry must be of the same species and thickness. The face and back panels are permitted to be of different species.
For plywood to conform to both of these basic principles, it must have an odd number of plies. Therefore, plywood is typically constructed with odd numbers of plies, such as three, five, or seven. The names of the plywood layers are: the surface veneer is called the faceboard, the inner veneer is called the coreboard; the front faceboard is called the topboard, and the back faceboard is called the backboard. The coreboard with fibers parallel to the faceboard is called the long core board or middle board. When assembling the cavity board blank, the top and backboard must face outward.
