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Plywood is a manufactured wood product that is made up of an odd number of thin sheets of wood. The sheets are placed, one over the other, in such a fashion that grains in alternate layers run at right angles to each other. This gives the plywood strength in both directions, whereas normal wood would have the strength only across the grain. An odd number of sheets are used so that the top and bottom sheets have grains facing in the same direction.
Plywood can be used for building projects for the interior of homes, for exterior work, as structural components of a structure or for marine purposes. As a building material, plywood offers strength, durability, lightness and rigidity. Well made plywood would also have a resistance to warping and splitting that can affect normal wood. Its availability in standard sizes allows for cost efficiency, and quite often designers and architects would take this into account while suggesting dimensions and shapes.
Image Source: Wikipedia: Use of plywood