A many-to-many reference can relate one record in either table to many records in the other table.
The only way to create many-to-many (N:M) reference between two tables in PostgreSQL is using an intermediate table, which records set association between primary keys of first table and primary keys of second table.
Database Designer for PostgreSQL helps you set many-to-many reference between two tables easily:
How to create...
To create a many-to-many reference between two tables, click the icon on the Palette toolbar. Your mouse cursor will change its appearance. Then consistently click two tables you want to link with many-to-many reference.