A "rooted tree" is a tree in which one vertex has been designated the "root". The edges of a rooted tree can be assigned a natural orientation, either "away from" or "towards" the root, in which case the structure becomes a "directed rooted tree". 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 T which is a subgraph of some graph "G" is a normal tree if the ends of every T-path in "G" are comparable in this tree-order . Rooted trees, often with additional structure such as ordering of the neighbors at each vertex, are a key data structure in computer science; see tree data structure.