Autofill formula changing

[ryanegger]

An easy one for all you gurus out there. I have the following formula:

=sum(a1+b1)

I want to drag it down and have it pattern like this

=sum(a2+b1)
=sum(a3+b1)
ect.

However, it goes:

=sum(a2+b2)
=sum(a3+b3)

How do I prevent this. Thanks for answering my dumb question...

[shg]

In the first cell, =a1+b$1

See Help for About cell and range references