How to auto-fill a county field based on zip code?

Hi again!
I came across this question because I am trying to do the same thing.
I have a table of zip codes and counties (fortunately I only need Massachusetts).
I set the zip code as the key field.
I created a relationship between this table and the Contacts table (where the address is with the zip code I want to match up).

Now I'm stuck. How do I tell it to match up the zip code in the address field with the zip code in the related counties table?

np,
Look at your address field properties. It will give you the name of the subfields.

Then make a new formula text field called [zip code] with a formula like

[Address: Postal Code]

Now you have a Text field eligible to be in a relationship.

Ok - I did that.
I'm still not sure how to match them up. Maybe I'm being dense because it's Friday afternoon. What am I missing?

Happy Friday,

Well the One side of the relationship will the the Key field of the Zip code table which I hope is the zip code.

Th eMany side needs to be that new field you just set up in the Contact table.  If you have a relationship just based on like [Related Zip code}, then you will need to kill it and start over or change that field to be of type formula text and make the formula

[Address;Postal Code]

Aha! Got it now! Thanks again for your patient and expert help. :)