In a tree, which is a special kind of graph, a "root" data item has associated with it some number of data items which in turn have associated with them some number of other data items in what is frequently viewed as a "parent-child relationship". Every data item (other than the root) has a single parent (the root has no parent) and some number of children, possibly zero. Examples of operations on trees are the addition of data items so as to maintain a specific property of the tree to perform sorting, etc. and traversals to visit data items in a specific sequence.