You can transform a relationship between two entities into an associative entity linked by two relationships, and then attach entity attributes to the associative entity that you could not attach to the relationship.
- Right-click a relationship symbol and select  . The original relationship is split in two and an associative entity is created between the two new relationships,  taking the name and code of the original relationship. 
- Open the property sheet of the associative entity or one of the new relationships to modify their properties as appropriate.