It was mentioned earlier that there is in fact a distinction between relationships and relationship
types. In Fig. 3.5(a) and (b) there are in fact two relationships shown: one between customer 2
and invoice 7 and one between customer 2 and invoice 9, so strictly speaking `received' is a
relationship type consisting of a number of relationships between entity types. However, this
distinction is sometimes dropped (it frequently is in this book), and both are given the name
`relationship'.