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.