 When a directed rooted tree has an orientation away from the root, it is called an "arborescence" or "out-tree"; when it has an orientation towards the root, it is called an "anti-arborescence" or "in-tree".The "tree-order" is the partial ordering on the vertices of a tree with if and only if the unique path from the root to "v" passes through "u".A rooted tree which is a subgraph of some graph "G" is a normal tree if the ends of every edge in "G" are comparable in this tree-order whenever those ends are vertices of the tree .