	The key difference between an AVL tree and a WAVL tree arises when a node has two children with the same rank or height. In an AVL tree, if a node has two children of the same height as each other, then the height of must be exactly . In contrast, in a WAVL tree, if a node has two children of the same rank as each other, then the rank of can be either or . This greater flexibility in ranks also leads to a greater flexibility in structures: some WAVL trees cannot be made into AVL trees even by modifying their ranks, because they include nodes whose children's heights differ by more than one.