Directed Graph (igraph)

Cited from the Book "Statistical Analysis of Network Data with R"

A graph G for which each edge in E has an ordering to its vertices (i.e., so that (u,v) is distinct from (u,v), for u,v ∈ V) is called a directed graph or digraph. Such edges are called directed edges or arcs, with the direction of an arc (u,v) read from left to right, from the tail u to the head v. Note that digraphs may have two arcs between a pair of vertices, with the vertices playing opposite roles of head and tail for the respective arcs. In this case, the two arcs are said to be mutual.