R. Bruce Hoadley wrote that grain is a "confusingly versatile term" with numerous different uses, including the direction of the wood cells (e.g., straight grain, spiral grain), surface appearance or figure, growth-ring placement (e.g., vertical grain), plane of the cut (e.g., end grain), rate of growth (e.g., narrow grain), and relative cell size (e.g., open grain).
Perhaps most important physical aspect of wood grain in woodworking is the grain direction or slope (e.g. against the grain). The two basic categories of grain are straight and cross grain. Straight grain runs parallel to the longitudinal axis of the piece. Cross grain deviates from the longitudinal axis in two ways, spiral grain or diagonal grain. The amount of deviation is called the slope of the grain.
In describing the application of a woodworking technique to a given piece of wood, the direction of the technique may be:
Grain alignment must be considered when joining pieces of wood, or designing wooden structures. For example, a stressed span is less likely to fail if tension is applied along the grain, rather than across the grain. Grain direction will also affect the type of warping seen in the finished item.
In describing the alignment of the wood in the tree a distinction may be made. Basic grain descriptions and types include: