Some of graph families love to have direction. They put an arrow in every edge. These graph families are called Directed Graph .
An arrow in directed graph must be supported by at least one vertex.
Normally it is supported by two vertices.
These tutorial is copyrighted.
Preferable reference for this tutorial is
Teknomo, Kardi (2015) Pictorial Introduction to Graph Theory. http:\\people.revoledu.com\kardi\ tutorial\GraphTheory\