plywood

Plywood is a sheet building material made by gluing thin layers of wood, called plies, with the grain of each layer running at right angles to the one below. That cross-grain construction is what makes it strong in both directions and far more stable than a solid board of the same thickness.

Because the plies alternate, plywood resists splitting along the grain, warps less as it gains and loses moisture, and holds a screw near its edge better than solid wood. The common grades are interior, exterior — waterproof glue, the basis of marine plywood — and the sanded or unsanded panels sold for shop work.

It is sold by thickness, with a quarter, a half and three quarters of an inch being the everyday sizes, and the odd number of plies, three, five or seven, is what keeps a sheet balanced and flat. An exposed edge is its weak point, so it is normally banded, capped or supported in use.