How does Access guess join field in query?

[bar tomas]

Hi,
I have a database in which I have not defined any relationships between tables.
However when I create a new query (in design view) and select two tables, Access shows a link between them on which the join can be made.


How does Access guess the field of each table on which to join? (the fields linked by Access do not have the same name in my database).
Thanks very much.

[RuralGuy]

I would not leave the type of join up to Access to guess.