When we have a vertex connected to an arc, we define:
- Its internal degree, denoted , as the amount of arcs that point to said vertex.
- Its external degree, denoted , as the amount of arcs that point away from said vertex.
Given a digraph with vertices, the sum of the internal degrees of all its vertices is equal to the sum of all the external degrees of all its vertices:
Both of these sums is equal to the amount of edges of the digraph.