Autofill field based on other field entry

[Martin]

I have a form that lists multiple fields from a query and my end-user
wants to be able to change one field and have it automatically update
another field, but not change the rest of the form. What I have is a
combo box where you can select a Rank and another box, currently
unbound, that should display the appropriate paygrade after you select
the rank. I have the two listed in the same table right now and they
share a PKey, but I'm staring to think thats not the best way to go
about this. I'm not good with relationships so, if that's the
solution, any assitance that can be provided will be greatly
apreciated.

Thanks in advance
Martin