Each Relationship Type Has Two Attendant Collection Types

There is a relationship between collection type and relationship type. The following figure shows some relationship types and their attendant collection types.

In the figure, each relationship type has exactly two collection types. The following are true for every relationship:

• Each relationship is a member of two collections.
  
  
• Each relationship relates two objects.
  
  
• You can read each relationship in two directions.
  
  
• In any relationship, the related objects participate in two separate roles. For example, the roles in the relationship Kraig Brockschmidt wrote Inside OLE, are:
  • The role of writing thing, filled by the object describing Kraig Brockschmidt.
    
    
  • The role of written thing, filled by the object describing Inside OLE.

  The two roles correspond to the two collection types.