\begin{definition}[Definition:Directed Graph/Simple Digraph]
Let $D = \struct {V, E}$ be a digraph.
If the relation $E$ in $D$ is also specifically asymmetric, then $D$ is called a '''simple digraph'''.
That is, in a '''simple digraph''' there are no pairs of arcs (like there are between $v_1$ and $v_4$ in the diagram above) which go in both directions between two vertices.
\end{definition}