I'm not sure if SUMIF's behavior was intended or not....but, I consider it a
nice keystroke-saving feature.  Since there is a one-to-one relationship
between the test cells and the sum cells, it makes sense that the 2 ranges
must be the same size. SUMIF extends the 1-cell third argument in the same
direction as the 1st range (horizontal or vertical).

Oddly, LOOKUP seems to have similar, but different behavior.  If it's second
argument is a vertical range and you only enter a single cell address for the
third argument....it assumes that range is horizontal! However, if you supply
a 2-cell vertical range for the third argument....it implicitly extends that
range downward.

I hope that helps.
***********
Regards,
Ron

XL2003, WinXP

Jason,

There is one very serious problem with this technique.

Let's take data of

...|    A|    B|    C|
.1|       |    x|     1|
.2|       |    y|     2|
.3|       |    x|     3|
.4|       |    y|     4|
.5|       |    x|     5|
.6|       |    x|     6|
.7|       |    x|     7|

and a formula of

=SUMIF(B1:B7,"x",C1)

On first input, this correctly returns 22.

However, on Excel 2000, if you change C7 to a value of 8, the formula does
not update. I assume that this is because C7 is not referenced in the
formula, so Excel's dependency algorithm doesn't kick in to force a
recalculation.

This had changed in Excel 2002/XP, where it automatically updates.
Presumably, this means that SUMIF has become a volatile function in 2002/XP?
Or maybe it internally stores the extended range. My testing suggests that
it is now volatile.

So, using a technique that behaves differently in different versions of
Excel is not a good idea IMO.