Basically, the easy answer is that -- is the same as multiplying the result
by -1 twice. So why do this?

ISNUMBER(C5) returns either TRUE or FALSE. Neither of which are usable by
SUMPRODUCT. SUMPRODUCT needs numbers. Excel does equate FALSE to 0 and TRUE
to 1. So, by multiplying TRUE or FALSE by -1 twice you get:

0 x -1 x -1 = 0
1 x -1 x -1 = 1

You now have either a 0 or a 1 that SUMPRODUCT can work with.

HTH
Elkar


"NWO" wrote:

Can somebody please explain what the puprose is of the -- in the following
formula?

=SUMPRODUCT(--(ISNUMBER(C5:C61)),--(MONTH(C5:C61)=2))

Thank you.

Mark