The Atlas of Historical County Boundaries http://www.newberry.org/ahcbp/index.html  is a useful tool for those seeking records of past events, trying to interpret historical data, or working on state and local history. Counties have been the principal geographic units for collection of materials including records of births, marriages, deaths, probated wills, real estate transfers and voting. Knowing the county name and its boundaries at specific points in time can be very significant. For the state of Minnesota, the county boundaries are mapped from 1778 to 2000. The ability to layer modern boundaries and names over historic ones, change the date of the map, zoom-in and zoom-out and measure distances between two locations are some of the useful features of the database.