Binary Relations among Elements in a Set
Binary asymmetric relations connecting two elements in a set are indicated by red arrows. This Demonstration shows the domain (light blue), range (or image) (light red) or (green) associated with such a relation. Blue arrows show the composition of the relation with itself ().
 E. J. Borowski and J. M. Borwein, The HarperCollins Dictionary of Mathematics, New York: HarperPerennial, 1991 p. 489.