Several years ago, I asked my great aunt why some of her siblings were born in Iowa, and some where born in Missouri? Her response surprised me, "We never moved," she said, "the state line moved!" Now, I had never considered the idea that state lines or county lines could move, but they most certainly did.
Recently, I became aware of an very useful website, the Atlas of Historical County Boundaries, that helps one visualize where these borders were, and where they are today. According to their website:
The map was produced by the Atlas of Historical County Boundaries Project, which has been supported by grants from the National Endowment for the Humanities, an independent federal agency. It also has benefited greatly from support from The Newberry Library, the project's headquarters and sponsor, software grants from Environmental Systems Research Institute (ESRI), and contributions from foundations and individuals.
It is very user friendly, and I spent quite some time just playing with the maps. Give it a try!