In practice it is wise to replace many-many relationship types such as this with a set (often two) of one-many relationship types and a set (often one) of new, previously hidden entity types. This is covered in a later section in this chapter.
3.6.2.4 Deriving a One-One relationship type
Fig. 3.15 gives an example of a one-one relationship type being derived. It concerns a person and his or her birth certificate. We assume that everyone has one and that a certificate registers the birth of one person only.