Recently, while browsing Lincoln Mullen’s website for advice on certain mapping software, I came across the Atlas of Historical County Boundaries. This resource is fantastic! Not only does it contain detailed files with the county and state borders from the early colonial period to 2010, but it also has incredible documentation on where it found the information of those borders. These long chronological lists of changes to the borders are complete with full citations. Anyone planning to either build a base map with historical boundaries or to examine how those boundaries changed over time will have a much easier time using this resource.