Well, If a check is discontinuous and not excessively large...I guess it's easier to show someone, but sufficed to say, "normal" checking is not considered a defect in timbers, and there is no need to reduce the rated strength of the timber. Checking is a natural part of the drying process whereby local stress is relieved. It occurs from uneven drying/shrinking of the timber. It is desirable to minimize it as much as possible. White pine will can commonly have timber with no significant checking, but hardwoods will usually have some degree of checking. Oak for example will usually check to some degree. We minimize checking by sealing the end grain with a wax based sealer (Anchorseal is a common brand used). The end grain of a piece of wood is the exposed capillaries of the wood. Water will run out/dry out very quickly from the ends if not sealed. This will lead to uneven drying/shrinking. Sealing those ends stops water from leaving allowing drying to only occur from the sides of the timber. Water will dry from the sides much more slowly and evenly. Slowing the drying and making it more even will allow the timber to shrink more uniformly and therefore reduce the internal stresses that lead to checking. When working with green wood, always seal the end grain, whether it's the end of the log, or the end grain within the mortice you just finished cutting.
