This form illustrates how to restrict the value of one combo box based on the value of another. In this case, you choose the city, and you get a listing of just the zip codes available for that city.

It also shows how to get the asterisk (*) to show up in the RecordSource for the city, so you can choose all cities. When you choose the asterisk to list all zipcodes and then choose a zip, the corresponding City appears in the city control.