A directed graph, also known as a digraph, is a graph in which each edge is replaced by a directed edge, indicated by an arrow. A directed graph having no multiple edges or loops is called a simple directed graph. A complete graph in which each edge is bidirected is called a complete directed graph. A directed graph having no symmetric pair of directed edges (i.e., no bidirected edges) is known as an oriented graph. A complete oriented graph (i.e., a directed graph in which each pair of vertices is joined by a single edge having a unique direction) is called a tournament or tour.