The wood grading rules vary greatly from one species of wood to another. There are several types of wood, and each has a different classification system based upon the way the wood is treated during its growth cycle. There are literally thousands of grades, which range from "dust" (low grades) to "medium" (medium grade), to "vitreous" (high grade) to "hardwood" (very high grade). Each type of wood has many variations in texture, color, grain, density, and other properties, learn at Baird. The classification systems are complicated and involve a lot of study and consideration.
The most important consideration in the classification of wood is species. The hardwoods grading guidelines are generally very complex and require careful study and consideration. A simple, condensed outline of the process for grading a particular piece of wood is as follows:
Determine which species the wood came from. There are many species of wood, and some of these species are more difficult to work with than others. For example, maple trees are easier to work with than oak trees. Look at the wood and make sure that it can be properly cut. If there are defects, make sure that the flaws are not so severe that the wood will split or come apart when it is cut.
Surface height (SH). SM is usually the width, in millimeters, of a single board, but some wood species may have multiple boards per foot, depending upon their location in the tree. SM is generally equal to twice the height of one side of a board. For instance, a two-foot wide board would have a SH of two inches. This means that the horizontal distance between the two pieces of a board would be two inches, and the vertical distance between the two pieces would be two inches. In general, SH is less than twice the height of one side of a board in most wood species.
Shape and grain. This refers to how each piece of wood has its own unique characteristics. Usually, the grain of the wood is straight and parallel to the grain of the wood. Other wood species have ridges and dips, see more at Baird Brothers. When cutting down a piece of wood to size, it is important to make sure that the cuts are consistent from side to side and board to board. Also, the cuts should be consistent and even in width across all boards and joists.
Grain direction. This refers to the angle the grain of the wood takes as it comes off the planer. A sloping piece of wood usually takes an angle of approximately forty to sixty degrees with respect to each other. Straight grain takes an angle of ninety to ninety-five degrees. Smooth grain takes an angle of about sixty-five degrees. A piece of wood that is broken up is usually of an angle of more than ninety degrees. The angles are usually in terms of either clockwise or counterclockwise. Read more at https://en.wikipedia.org/wiki/Wood