In order to improve the anisotropic characteristics of natural wood as much as possible, so that the characteristics of plywood are uniform and the shape is stable, generally plywood must comply with two basic principles in structure: one is symmetry, and the other is that the adjacent layers of veneer fibers are perpendicular to each other. The principle of symmetry is to require that the veneer on both sides of the symmetrical center plane of the plywood should be symmetrical with each other regardless of the nature of the wood, the thickness of the veneer, the number of layers, the direction of the fiber, the moisture content, etc. In the same plywood, veneers of a single species and thickness may be used, or veneers of different species and thicknesses may be used, but any two layers of veneer species and thickness that are symmetrical to each other on either side of the symmetrical center plane should be the same. The backing of the face is allowed not to be of the same species.
For the structure of plywood to conform to both of the above basic principles, its number of layers should be odd. Therefore, plywood is usually made into three layers, five layers, seven layers and other odd layers. The name of each layer of plywood is: the surface veneer is called the surface board, the inner layer of the veneer is called the core board, the front surface board is called the panel, the back of the surface plate is called the back plate, and in the core board, the fiber direction parallel to the surface plate is called the long core board or the middle board. When composing the cavity table slab, the panel and backplate must be facing outward.
