LEFT JOIN problem

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