LEFT JOIN problem

Hi
Extreme novice here. Pls be gentle. I have a database with four tables

orders (have a foreign key to depots.de_id)
depots (have a foreign key to customers.co_id)
companies
addresses

depots have an optional cross reference from depots.de_ad_id to
addresses.ad_id

so:

SELECT co_id, de_id, or_id
FROM orders, depots, companies
LEFT JOIN addresses ON (de_ad_id=ad_id)
WHERE or_id=de_or_id AND co_id=de_co_id;

returns error
ERROR: invalid reference to FROM-clause entry for table "depots"
LINE 1: ...pots, orders, companies LEFT JOIN addresses ON (depots.de_...
^
HINT: There is an entry for table "depots", but it

The error message is incomplete.

Any idea whats wrong with my code? Thanks

Mike