This relation is reflexive and symmetric. The graph for this tolerance relation would involve five vertices (five elements in the relation), as shown in Figure 3.9. The property of reflexivity (diagonal elements equal unity) simply indicates that a city is totally related to itself. The property of symmetry might represent proximity: Omaha and Chicago (x1 and x2) are close (in a binary sense) geographically, and Chicago and Detroit (x2 and x5) are close geographically. This relation, R1, does not have properties of transitivity, for example