In order to improve the anisotropy of natural wood as much as possible, the plywood has uniform characteristics and stable shape. Generally, the plywood must follow two basic principles in structure: one is symmetrical; the other is that the adjacent single-board fibers are perpendicular to each other. The principle of symmetry is to require veneers on both sides of the symmetry center plane of the plywood, regardless of the nature of the wood, the thickness of the veneer, the number of layers, the direction of the fibers, the moisture content, etc., should be symmetrical to each other. In the same plywood, a single tree and a thickness of veneer can be used, or a veneer of different tree species and thickness can be used; however, any two symmetrical veneer species and thicknesses on either side of the symmetrical central plane are the same. The back panel allows not the same species.
In order for the structure of the plywood to conform to the above two basic principles, its number of layers should be an odd number. Therefore, plywood is usually made into three layers, five layers, seven layers and other odd number of layers. The names of the layers of the plywood are: the surface veneer is called the table, the inner veneer is called the core board; the front panel is called the panel, and the back panel is called the back panel; in the core board, the fiber direction is parallel to the panel. It is called a long core board or a medium board. When forming the cavity plate slab, the panel and the back plate must be facing outwards.