Each circle represents a node. Each node contains three bits of information

 

Taking the first node of the tree we have Martin as the first node of the tree. This is called the root. Hanging off from the root is two other nodes, one on the left and one on the right. A node which has nothing connected to it is known as a leaf. Nodes, and all of the other nodes connected to them, are called sub-trees.

In this case there are two sub trees connected to Martin; George and Nora. Look carefully at the order of the nodes as it is important. Binary trees are only binary trees if the following rules hold –

George appears lower in the alphabet than Martin which is why it is found on the left, it is less than. Nora appears on the right as she is greater than Martin. These two rules must always hold for all nodes of the tree. If you look over the whole tree on the last page you should be satisfied that this is the case.