All lumber is divided as a matter of convenience into two great groups: softwoods and hardwoods. The softwoods in general are the coniferous or cone-bearing trees (such as the various pines, spruces, hemlocks, ?rs, and cedar). The hardwoods are the non–cone-bearing trees (such as the maple, oak, and poplar). These terms are used as a matter of custom, for not all so-called softwoods are soft, nor are all so-called hardwoods necessarily hard. In fact, such softwoods as long-leaf southern pine and Douglas ?r are much harder than poplar, basswood, and so on, which are called hardwoods.

Other (and perhaps more accurate) terms often used for these two groups are the needle-bearing trees (softwoods) and the broad-leaved trees (hardwoods). In general, the softwoods are more commonly used for structural purposes such as for joists, studs, girders, and posts, whereas the hardwoods are more likely to be used for interior ?nish, ?ooring, and furniture. The softwoods are also used for interior ?nish and, in many cases, for ?oors, but are not often used for furniture.

A tree consists of the following:

• Outer bark — The bark is living and growing only at the cambium layer. In most trees, the bark continually sloughs away.
• Inner bark — In some trees (notably hickories and basswood) there are long tough ?bers (called bast ?bers) in the inner bark. In other trees (such as the beech), these bast ?bers are absent.
• Cambium layer — This can be only one cell thick. Only these cells are living and growing.
• Medullary rays or wood ?bers — These run radially from the center to the bark.
• Annual rings — These are layers of wood.
• Pith — This is at the very center.

Around the pith, the wood substance is arranged in approximately concentric rings. The part nearest the pith is usually darker than the parts nearest the bark and is called the heartwood. The cells in the heartwood are dead. Nearer the bark is the sapwood, where the cells carry or store nutrients but are not living.

Cross-section of an oak nine years old, shows pith, concentric rings comprising the woody part, the cambium layer, and the bark. The tree grows in concentric rings (or layers) with one layer added each year. The rings are also called annual rings.

As winter approaches, all growth ceases. Each annual ring is separate and, in most cases, distinct. The leaves of deciduous trees (or trees that shed their leaves) and the leaves of some of the conifers (such as cypress and larch) fall, and the sap in the tree may freeze hard. The tree is dormant but not dead. With the warm days of the next spring, growth starts again strongly, and the cycle is repeated. The width of the annual rings varies greatly, from 30 to 40 or more per inch in some slow-growing species, to as few as 3 or 4 per inch in some of the quick-growing softwoods.