A tree is a non-empty set one component of which is designated the root of the tree while the remaining components are partitioned into non-empty groups each of which is a subtree of the root.

Tree nodes have much useful functionality. The depth of a node is the length of the path from the root to that edge. The height of a node is the longest way from that node to its leaves. The length of a tree is the height of the parent node. A parent node has no children -- its only path is up to its parent. Describe the axiomatic development of trees and its rules for more information.