Simulating a natural join in ISQL

I have two tables that I wish to join without losing any data. The first has a number of rows, all with an order number and line number. This is the primary key. A second table has other information, but also has the same primary key. However with this table not all order number line numbers appear in it. If I join with a select on equality of primary keys I'll lose the rows that have order number line numbers in the first table but not in the second. I need to get around this problem. Any suggestions?