Fig. 3.7 There can be one, two, three or more entity types involved in a relationship.
It is sometimes possible to replace higher-order relationship types (ternary and above) by a collection of binary relationship types linking pairs of the original entity types. However this is not always possible (although as we shall see, in 3.6.1.1 below, the high-order relationship can always be redefined, with suitable renaming, as an entity type). In the example cited above concerning lecturers recommending textbooks on courses, it is not possible to replace the ternary relationship type 'recommends' with two or even three binary relationship types because information would be lost.