Graphs can be used to model a variety of things such as

• study groups in a class,
• showing genetic relationships between organisms,
• sudoku puzzles,
• committee assignments, and
• computer program dependencies.

If we make the edges of a graph point in only one direction then we get what is called a digraph (or a directed graph). A digraph is perfect for displaying a heirarchy of objects. For example, we can use a digraph to see the math courses and prerequisites at NSC. In the digraph below the class numbers are the vertices, and one class points to another if it is listed as a prerequisite in the NSC course catalog.