Juvenile wood, which is wood formed near the centre of a tree, shrinks more longitudinally than mature wood. Reaction wood, which is formed in parts of Ieaning or crooked stems and in branches, also shrinks more longitudinally than normal wood. When one edge or face of a board contains normal wood and the opposite edge or face contains either of the two forms of abnormal wood, the difference in shrinkage along the length of the board causes crook or bow. Grain deviations on the edges or faces of boards can also cause unbalanced longitudinal shrinkage. Cross grain, where the wood fibres are not lined up with the long axis of a board, can also lead to unbalanced shrinkage.
Knots are another source of localized grain deviation that can cause crook or bow.
Twist is a form of warp caused by a combination of unequal longitudinal shrinkage und grain deviations such as spiral, wavy, diagonal, locally distorted, or interlocked grain. Diamond is a form of warp found in squares. It results from the difference between radial and tangential shrinkage in squares where the growth rings run diagonally from corner to corner. Oval occurs in rounds, and is also caused by the difference between radial and tangential shrinkage. The other cause of warp, growth stresses, is not related to drying but can nevertheless be serious.