This strengthened condition allows us to delete a key from the tree in one downward pass without having to "backup" (with one exception, which we'll explain.) The following specication for deletion from a B-tree should be interpreted with the understanding that it ever happens that the root node x becomes an internal node having no keys (this situation can occur in cases 2c and 3b, below), then x is deleted and x's only child c1[x] becomes the new root of the tree, decreasing the height of the tree by one and preserving the property that the root of the tree contains at least one key (unless the tree is empty.)