Tree structure

A tree structure is a way of representing the hierarchical nature of a structure in a graphical form. It is named a 'tree structure', because the graph looks a bit like a tree, even though the tree is generally shown upside down compared with a real tree, that is to say with the root at the top and the leaves at the bottom.

Illustration: A tree structure showing the possible hierarchical organization of an encyclopedia. This specific example happens to a binary tree, which means all nodes have exactly zero or two child nodes.

The lines connecting elements are called branches, the elements themselves are called nodes. Nodes without children are called end-nodes or leafs.

The names of relationships between nodes are modelled after family relations. In computer sciences, traditionally only names for male family members have been used. In linguistics, the names of female family members are used. It is said that this was an express counter movement to the traditional naming convention, started by the female students of linguist Noam Chomsky.

The starting node is often called the root.

A node is a parent of another node, if it is the node one step higher up in the hierarchy and closer to the root node.

A node is a sibling (brother or sister) of another node, if they have the same parent nodes.

In the example, 'encyclopedia' is the parent of 'science' and 'culture', its children. 'Art' and 'craft' are siblings, and children of 'culture'.

A given node is considered to be an ancestor of all the lower level nodes to which the given node is connected.

Tree structures are used to depict all kinds of taxonomical knowledge, such as family trees, the grammatical structure of a language (the famous example being S -> NP VP, meaning a sentence is a noun phrase and a verb phrase), the way web pages are logically ordered in a web site, et cetera.

Trees have a number of interesting properties:

• The root node, i.e., the base node, is an ancestor of all the other nodes.
• In a tree structure there is one and only one path from any point to any other point.

Tree structures are used extensively in computer science and telecommunications.

See: tree graph, hierarchical tree structure, tree data structure, B tree, Tree of life, rooted hierarchical tree structure.