The Atlas of Historical County Boundaries is a free site hosted by The Newberry Library for understanding how state and county boundaries have changed through the years, helping genealogists trace family records through historical boundary changes.

“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. It also includes non-county areas, unsuccessful authorizations for new counties, changes in county names and organization, and the temporary attachments of non-county areas and unorganized counties to fully functioning counties. The principal sources for these data are the most authoritative available: the session laws of the colonies, territories, and states that created and changed the counties.”

“A dozen features distinguish the volumes and files of this atlas from other compilations.

1. All boundary changes in states and counties-unrivaled historical and geographic coverage.

2. Non-county areas-never before compiled or mapped.

3. Attachments to operational counties (non-county areas and unorganized counties)-never before compiled or mapped.

4. Separate map or polygon for every different county configuration-clarity and ease of use.

5. Based on original research in primary sources-unlike most reference works.

6. Primary sources cited for every change-unmatched documentation.

7. Information organized by both date and county-unmatched flexibility.

8. Locator maps for all county maps-show each county’s location within its state.

9. Area (sq. mi.) for each county configuration-available nowhere else.

10. Polygons available in two formats: shapefiles and KMZ-broad applicability.

11. Supplementary bibliography, chronologies, and commentary-unusually complete and thorough data presentation.

12. Short and Long metadata documents for each state dataset-convenience and completeness.”