In the above graph there is at most one edge joining each pair of vertices. Suppose
now, that in Fig. 1.5 the roads joining Q and S, and S and T, have too much traffic to
carry. Then the situation is eased by building extra roads joining these points, and the
resulting diagram looks like Fig. 1.6. The edges joining Q and S, or S and T, are called
multiple edges. If, in addition, we need a car park at P, then we indicate this by drawing
an edge from P to itself, called a loop (see Fig. 1.7). In this book, a graph may
contain loops and multiple edges. Graphs with no loops or multiple edges, such as the
graph in Fig. 1.5, are called simple graphs.