Weak branch unions are places where branches are not solidly attached to the tree. A weak union occurs when two or more similarly sized, usually upright branches, grow so closely together that bark grows between the branches, inside the union. This ingrown bark does not have the structural strength of wood and the union is much weaker than one that does not have included bark. The included bark may also act as a wedge and force the branch union to split apart. Trees with a tendency to form upright branches, such as elm and maple, often produce weak branch unions. Weak branch unions also form after a tree or branch is tipped or topped; when the main stem or a large branch, is cut at a right angle to the direction of growth leaving a large branch stub. The stub inevitably decays, providing very poor support for new epicormic branches that usually develop along the cut branch.