If a relationship type is between entities in a single entity type then it is called a unary relationship type. One example is the relationship `friendship' between entities within the entity type PERSON. If a relationship type is between entities in one entity type and entities in another entity type then it is called a binary relationship type because two entity types are involved in the relationship type. An example is the relationship 'Received' in Fig. 3.4 and Fig. 3.5 between customers and invoices. Another example of a binary relationship type is 'Purchased' between entity types CUSTOMER and PRODUCT. Two entity types are involved so the relationship is binary.