To minimize the anisotropic properties of natural wood and ensure uniform characteristics and shape stability in plywood, two basic principles must be followed in its structure: symmetry and perpendicularity of veneer fibers between adjacent layers. The principle of symmetry requires that the veneers on both sides of the symmetrical central plane of the plywood be symmetrical regardless of wood properties, veneer thickness, number of layers, fiber direction, or moisture content. A single plywood sheet can use veneers of a single species and thickness, or veneers of different species and thicknesses; however, any two symmetrical layers on either side of the symmetrical central plane must have the same species and thickness. The face and back veneers can be of different species.
For the plywood structure to simultaneously meet both basic principles, its number of layers must be odd. Therefore, plywood is typically made with three, five, seven, or other odd-numbered layers. The names of the layers in plywood are as follows: the top veneer is called the face veneer, and the inner veneer is called the core veneer; the front face veneer is called the face veneer, and the back face veneer is called the back veneer; among the core veneers, those with fibers parallel to the face veneer are called long core veneers or middle core veneers. When assembling the cavity board blank, the face veneer and back veneer must have their tight sides facing outwards.
