A **binary tree** is an ordered tree data structure in which each node has at most two children. Typically the child nodes are called *left* and *right*. One use of binary trees is as binary search trees.

There is a one-to-one mapping between general ordered trees and binary trees which Lisp uses to represent general ordered trees as binary trees. Each node N in the ordered tree corresponds to a node N' in the binary tree; the *left* child of N' is the node corresponding to the first child of N, and the *right* child of N' is the node corresponding to the N's next sibling --- that is, the next node among N's parent's children.

One way of thinking about this is that each node's children are in a linked list, chained together with their *right* fields, and the node only has a pointer to the beginning of this list.