Wood is broadly divided into hardwoods and softwoods, but the names refer to botanical origin rather than true hardness. Hardwoods come from deciduous trees such as oak, maple, walnut, cherry, mahogany, ash, birch, and beech, while softwoods come from conifers like pine, fir, cedar, spruce, and redwood. The Janka hardness test measures resistance to denting in pounds-force, and the category labels can mislead: balsa is technically a hardwood despite being one of the softest woods around.
The structure of wood is visible in its grain, the pattern of fibers that affects both appearance and behavior. Grain direction matters because cutting with the grain yields a cleaner cut, while cutting against it tends to tear the fibers. End grain shows the cross-section of fibers at cut ends, face grain shows the broad sides, and edge grain shows the narrow side with fibers running parallel. Some woods display figured patterns such as bird's eye (small swirls, especially in maple), curly figure (wavy bands of light and dark), and quilted figure (a three-dimensional bumpy look in light). Growth rings record the tree's age and growing conditions, while the inner heartwood is denser and often darker, and the outer sapwood is lighter, having conducted the tree's sap. Heartwood is generally more stable and decay-resistant than sapwood.
Wood is a hygroscopic material, meaning it constantly exchanges moisture with the air. This movement occurs primarily perpendicular to the grain: tangential movement across the rings is the greatest, radial movement across the radius is roughly half that, and longitudinal movement along the grain is negligible. Good design accounts for this expansion and contraction rather than fighting it. For typical indoor furniture, an equilibrium moisture content of about 6 to 9 percent is desirable, while outdoor wood equilibrates higher. A moisture meter verifies that wood has reached a suitable level before use, whether it has been kiln-dried in a controlled environment or air-dried slowly outdoors. The whole drying process is called seasoning.