A root knee is a side root that together with the lowest part of the trees trunk forms a very strong, angled piece of wood without any joint.
Spruce and pine growing on wet grounds often have useable rot knees, though any tree with the right shape can be used. Traditionally root knees have been used as brackets in buildings and shelves and to keep the turf log in place on sod roof. They have also been used in looms and in boatbuilding.
Root knees are still used in wood craft where strong, angled wood is needed and in boat- and house reconstructions.