At last...an on-line, interactive program to study the change in state and county boundaries over time. That is what the NewBerry Library's "Atlas of Historical County Boundaries" provides.
This is a feature-rich website that demands a lot of exploration to get the most out of it, but is intuitive enought that you can start making it work for you immediately.
Now you can trace how county lines changed over time (you select your state of interest and timeframe of interest), which of course, influenced in what courthouse your ancestor's records might be archived.
Check out the site at http://publications.newberry.org/ahcbp/