Plywood is the most popular and versatile wood based panels nowadays. It’s a laminated product made up of numerous thin strips of wood laid in alternating directions and bonded with different kind of glue. But due to its construction method, plywood is less susceptable to expansion or shrinkage. Why?
A board of made from solid wood is unstable and tend to expand or shrink across the grain based upon the moisture content in the wood. Because plywood is much stronger and less likely to expand or shrink along the grain, then it is much more likely to split with the grain than against.
By gluing numerous thin strips of veneers in alternating directions layer by layer, plywood is much less likely to expand or shrink in moisture environment, and the this structure is much stronger than a similarly sized board cut from a single tree.
Plywood is made from an odd number of layers, each layer called ply(typically 3, 5 or 7), with an equal number of plies on each side of the center ply. The plies face are parallel, and the grain of the surface plies follows the long side of the board(although not always). The sheet is strongest parallel with (as opposed to against) the two surface plies.