You normally create a join between the primary key in one table and the foreign key of another table. You can also create a join between two primary keys. It is very unusual for at least one side of a join to not include the primary key of the table.
You need to understand how each key is constructed in your database. Multi column keys can affect how you set cardinalities for joins, and this can affect how you set up contexts in your schema.
Detecting and Using contexts is described in Detecting and Solving Join Problems.