 (Notice that while the \( sum \) of two binary relations is a binary relation, their \( product \) is a relation between pairs. This is the generalization from classes to relations of the fact that the cardinality of the relative product of two classes is the cardinality of the class of ordered pairs of elements taken one from each.) It is possible to prove results that show the differences between products and sums of relations and of numbers. The product of relations is associative: