Plywood is a panel product manufactured by glueing one or more veneers to both sides of a central veneer layer or a lumber-strip core. The development of plywood (as well as laminated and particle board) was made possible by the production of improved adhesives—especially synthetic resins—in the 1930s and ’40s. Most plywood is all-veneer; lumber-core plywood is produced only in small quantities. In both types of plywood products, the species, thickness, and grain direction of each layer are matched with those of its counterpart on the other side of the central layer. After the glue is spread, the panels are assembled and brought for pressing, usually in large, multistoried hot presses, where loading is automatic. Adhesives are thermosetting synthetic resins, phenol-formaldehyde is the most commonly used and urea-formaldehyde for plywood and can produce joints more durable than the natural wood itself and are thus highly resistant to weather, microorganisms, cold water, hot water, boiling water, seawater (“marine” plywood), steam, and dry heat. After pressing, the panels are stacked to cool and then sanded, graded, and stored. Plywood ranges in thickness from 4 mm for all-veneer to 32 mm for lumber-core.