An expression like:
a1:a4=1
will result in an array of 4 true/false elements.

so the -- stuff in this expression
--(a1:a4=1)
will coerce the trues to 1's and the false's to 0's.
-true = -1
--true = +1

This syntax is commonly used in array/sumproduct formulas.

Adjust the ranges to match--but you can't use whole columns (except in xl2007).

=sumproduct() likes to work with numbers. The -- stuff changes trues and falses
to 1's and 0's.

Bob Phillips explains =sumproduct() in much more detail he
http://www.xldynamic.com/source/xld.SUMPRODUCT.html

And J.E. McGimpsey has some notes at:
http://mcgimpsey.com/excel/formulae/doubleneg.html

dcstoo wrote:

What does the syntax in the subject denote? What is the purpose of "--"
(double dash)?

--

Dave Peterson