What happens when a "NULL" condition is applied to the

Hi, I have below two tables "TAB1" & "TAB2":

TAB1
----
T1C1 T1C2
---- ----
1 A
2 B
3 C

TAB2
----
T2C1 T2C2
---- ----
2 D
3 E
4 F

When I executed the below sql statement, it displayed the Cartesian product of the rows of both the tables, which I understood.
SELECT t1.t1c1, t2.t2c1
FROM TAB1 t1
LEFT JOIN TAB2 t2 ON t1.t1c1 IS NOT NULL;

But, when I execute the below sql statement, its displaying the below output, which I am not able to understand.
SELECT t1.t1c1, t2.t2c1
FROM TAB1 t1
LEFT JOIN TAB2 t2 ON t1.t1c1 IS NULL;

Output:
------
T1C1 T2C1
---- ----
1 null
2 null
3 null

Can anyone please explain me why its displaying the above output. It shouldn't have displayed any rows.
Thanks in Advance...

take a look in the help
http://msdn.microsoft.com/en-us/library/aa178429(SQL.80).aspx

and it's written: "It ALSO returns..."

try out and you'll probably see
SELECT t1.t1c1, t2.t2c1
FROM TAB1 t1
LEFT JOIN TAB2 t2 on t1.t1c1 ='1';

kr moby