<P> For a vertex, the number of head ends adjacent to a vertex is called the indegree of the vertex and the number of tail ends adjacent to a vertex is its outdegree (called "branching factor" in trees). </P> <P> Let G = (V, A) and v ∈ V . The indegree of v is denoted deg (v) and its outdegree is denoted deg (v). </P> <P> A vertex with deg (v) = 0 is called a source, as it is the origin of each of its outcoming arrows . Similarly, a vertex with deg (v) = 0 is called a sink, since it is the end of each of its incoming arrows . </P> <P> If a vertex is neither a source nor a sink, it is called an internal . </P>

A simple directed graph (also called a digraph) with indegrees 0 1 2 and outdegrees 0 1 2