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As you learn carpentry techniques, it's important to become familiar with the characteristics and uses of various types of lumber and how to choose the wood that will work best for a particular project.

There are two basic types of lumber: softwoods, typically made from coniferous trees, and hardwoods, made from deciduous trees. Wood is graded according to how many knots it has and the quality of its surface (see the chart in this section for the most common grades). Some lumberyards have their own grading systems, but they usually simply rename these standard grades.

No matter what species of lumber you buy, be on the lookout for the types of wood problems shown under Wood Defects. A board that is heavily twisted, bowed, cupped, or crooked usually is not usable, although some bows will lie down as you nail them in place. Knots are only a cosmetic problem unless they are loose and likely to pop out. Checking, which is a rift in the surface, also is only cosmetic. Splits cannot be repaired and will widen in time. Cut them off.

The nominal dimensions of wood are used when ordering lumber. Keep in mind that the actual dimensions of the lumber will be less (see Lumber Selection Tips). Large quantities of lumber are sometimes figured by the board foot. A board foot is the wood equivalent of a piece 12 nominal inches square and 1 inch thick (see Chart; Figuring Board Feet in this section). Most lumberyards will not require you to figure board feet.

The illustration shows some of the most common defects in lumber. Some of these problems can be overcome with proper construction techniques. In most cases, however, it is better to seek out a better quality of lumber from the start.