The Atlas of Historical County Boundaries is a powerful historical research and reference tool in electronic form. The Atlas presents in maps and text complete data about the creation and all subsequent changes (dated to the day) in the size, shape, and location of every county in the fifty United States and the District of Columbia.
This site contains over a thousand map images and direction to where others can be found. Also, the site is organized as a Checklist of Pennsylvania Maps to 1800 to provide a useful source tool. A little Pennsylvania history is woven into the descriptions and a list of cited references is included. Maps and commentary are added periodically, so check back if interested.