Knots are observable imperfections in the grain of a particular wood. If a board has been cut vertically from the trunk of a tree, the knot would look like a circular patch of abnormal wood which was once the part which a branch was attached to the trunk around which other rest of the grain has grown. This is exactly a true illustration of the occurrence that leads to the wood's deformity. The visible knot indicates the part of the trunk where such a branch was attached.

 

As trees grow taller, lower branches tend to wither and die due to an insufficient amount of sun reaching them as a result of obstructions by branches above. Because these branches are strong and large, they keep growing around the area rather than fall off instantly. This results in the conical shape off wood knots. Buds that are yet to blossom and old dead branches often result in dead spots on the wood which affects the next seasonal growth layer's contour. Knots are often of a harder texture due to the fact that the wood was compressed at that site and their grains run perpendicular to the remaining grains of the tree.