There can be more than one type of relationship between entities. For an example of three different relationship types between two entity types see Fig. 3.31. Fig. 3.4 shows a single relationship type 'Received' and its inverse relationship type 'Was_sent_to' between the two entity types CUSTOMER and INVOICE. It is very important to name all relationship types. The reader of the diagram must know what the relationship type means and it is up to you the designer to make the meaning clear from the relationship type name. The direction of both the relationship type and its inverse should be shown to aid clarity and immediate readibility of the diagram. The tense of the relationship type should also be clear from its name.