Knots on a tree are formed when branches—which are often smaller in size and located lower on the trunk—either break off or are cut off, and the outer layer or bark of the tree grows around or over the broken area. When the tree is cut, the knot is visible on the plank. It can appear as a dark circular shape surrounded by growth rings, which grew around the knot or broken branch. Knot sizes can vary depending on the wood species and the way the knot grew within the tree, as well as the angle that the planks were cut. Lumber with visible knots is graded according to its form, size, stability, and how firm it is when held in place.