Here's another explanation, from John McGimpsey's site:

http://www.mcgimpsey.com/excel/formulae/doubleneg.html

Hope this helps.

Pete

On Oct 1, 2:32*pm, "Michelle" wrote:
I don't understand why sometimes I have to use "--" before an argument in
the SUMPRODUCT fuction, and sometimes a "*".

Can anyone point me to a resource that will explain why and when I have to
use these instead of just commas, so that I don't have to post any more
questions about it!

Thanks

M