Let A be the set of students in your school, and let B be the set of courses.

Let A be the set of students in your school, and let B be the set of courses. Let R be the relation that consists of those pairs (a, b), where a is a student enrolled in course b. For instance, if Jason Good friend and Deborah Sherman are enrolled in CS518, the pairs (Jason Good friend, CS518) and (Deborah Sherman, CS518) belong to R. If Jason Good friend is also enrolled in CS510, then the pair (Jason Good friend, CS510) is also in R. However, if Deborah Sherman is not enrolled in CS510, then the pair (Deborah Sherman, CS510) is not in R.

Note that if a student is not currently enrolled in any courses there will be no pairs in R that have this student as the first element. Similarly, if a course is not currently being offered there will be no pairs in R that have this course as their second element.