Understood that perfectly! :D
So why don't I have to use ctrl+shift+enter since this is an array formula?

"Rowan" wrote:

The -- is a double unary minus which forces the Sumproduct function to
Convert True and False answers to 1's and 0's. More confused now?

There is a great explanation of Sumproduct at
http://www.xldynamic.com/source/xld.SUMPRODUCT.html

and further explanation of the double unary minus from Bob Phillips and
Harlan Grove at
http://tinyurl.com/bv42x

Hope this helps
Rowan