Wood grain is the pattern determined by the orientation of wood fibers and is formed when trees produce new cells during growth. Wood strength is the ability to withstand various kinds of stress, including impact and weight. The two are closely related. The strength of a piece of wood depends partly on the natural wood grain and partly on the way the wood is cut and used in relation to the wood grain. While there are over 1,000 kinds of trees in North America alone, only about 100 of them provide wood that is strong enough to use for construction.
If you look at a stump, you are viewing across the grain, at about a 90 degree angle. You will see tree rings recording the tree's annual growth. The fibers are aligned vertically up and down the trunk to provide a sort of skeleton, allowing the tree to grow upright. Boards are cut vertically, "with the grain," to provide the greatest strength. Trees that grew straight with relatively little side growth produce the strongest lumber. In the days of sailing ships, some tree species were harvested especially for use as masts because their growth patterns produced such a straight and even wood grain, resulting in great strength.
Even within one species, the environment can cause different growth patterns, resulting in different kinds of wood grain. The way the boards are cut from the log can also produce changes in wood grain. There are six standard descriptions.
In boards with straight grain, the board has been cut so that the wood fibers run up and down the length of the board. The board was cut along the length of the tree trunk, resulting in the greatest wood strength. In cross grain boards, the board was cut "across the grain," roughly perpendicular to the tree trunk. This results in the least wood strength, and boards are rarely cut this way on purpose.
Diagonal grain is found when a log is cut at an angle to make boards, instead of along the length of the log. Cutting diagonally across the wood grain reduces wood strength. A 45 degree angle, called a 1 in 1 slope of grain, reduces wood strength by 90 per cent. Even as little as 1 in 20 slope of grain causes a seven per cent reduction in wood strength.
Spiral grain is produced when a tree grows in a twisted way. Some species are more likely to produce twisted growth. The spiral is consistent, twisting to either the right or the left along the length of the log.
Wavy grain is produced when the direction of the wood fibers changes frequently, though usually by less then 45 degrees. The boards cut with wavy grain are often very attractive and are used for decorative projects.
Irregular grain is produced when some of the wood fibers change direction, but the frequency, direction, and degree of change is not regular. One example is the fiber around a knot, which moves out from vertical and then back in to allow room for the knot. These patterns can be attractive, and sometimes knotty wood with irregular wood grain is preferred for decorative uses. The placement of the irregular wood grain makes a difference to the strength of the wood. Knots make the wood more likely to crack and warp.