Attributes of Relationship Types

A student enrols in a course on a date, and later gets a grade.

Where does the grade go? Not on Student, because a student has one grade per course, not one grade. Not on Course either, for the same reason.

The grade belongs to the pairing. It is a fact about this student in this course, and nothing else.

An attribute that describes the link rather than either thing it connects is called an attribute of a relationship type. It is drawn as an ellipse hanging off the diamond.