Given sets $D_{1}, \ D_{2}, \ …, \ D_{n} $ not necessarily distinct.
The Cartesian product $D_{1}XD_{2}X.....XD_{n}$ is the set of all (ordered) $n-tuples$ $< d_{1}, \ d_{2}, \ d_{3}, \ ....... \, \ d_{n} >$ such that $d_{1} \ \epsilon \ D_{1} , \ d_{2} \ \epsilon \ D_{2} \ ..........., \ d_{n} \ \epsilon \ D_{n}$
A mathematical relation on $D_{1}, \ D_{2}, \ …, \ D_{n} $ is a subset of the Cartesian product $D_{1}XD_{2}X.....XD_{n}$
$D_{1}, \ D_{2}, \ …, \ D_{n} $ are the domains of the relations.