"Ken Wright" wrote...
Main thing to ensure is that all ranges are the same size, albeit there is
an exception to that when using TRANSPOSE to flip a range when testing
against each of the elements. You also cannot use whole column references
in your formulas.
....

It's not an exception. All Ranges must be the same size, but

=SUMPRODUCT(--(A1:A100=TRANSPOSE(B1:B5)))

has *ONE* argument, so it's necessarily the same size as itself. As for the

A1:A100=TRANSPOSE(B1:B5)

term, it's a derived array reflecting well-defined semantics. To wit, the
arrays on either side of the = are confomant, i.e., either MMULT(LHS,RHS) or
MMULT(RHS,LHS) evaluates numerically.