Many organizations and reference works follow the Census Bureau's definition for the region. In the history of the United States, the Mason–Dixon line between Pennsylvania (the North) and Maryland (the South) traditionally divided the regions, but in modern times, various entities define the Northeastern United States in somewhat different ways. The Association of American Geographers divides the Northeast into two divisions: "New England", which is the same as the Census Bureau; and it has the same "Middle States" but adds Delaware. Similarly, the Geological Society of America defines the Northeast as these same states but with the addition of Maryland and the District of Columbia. The narrowest definitions include only the states of New England. Other more restrictive definitions include New England and New York as part of the Northeast United States, but exclude Pennsylvania and New Jersey.