Northern Commonwealth

The Northern Commonwealth was one of the thirteen commonwealths of the pre-War United States. Created in 1969, it was comprised of the former states of Montana, Wyoming, North Dakota and South Dakota.

Former Wyoming
In 2281, the Great Khans could have abandoned the Mojave, and worked with the Followers of the Apocalypse in Wyoming to create a mighty empire. Also, the White Legs can flee to Wyoming after their leader, Salt-Upon-Wounds, is killed.

The region is described as a vast wilderness, and it experiences harsh winters.

Former Montana
Little is known about this area, except that there is a Brotherhood of Steel bunker here, the Lonesome Drifter was born here, there are coal mines active in the area and, if the Courier has the Lady Killer perk, it is revealed he may have had a child with a woman from there.

Former South Dakota
In Fallout 4, MacCready mentions a warehouse in South Dakota while searching warehouses in Goodneighbor, saying "Some strange things turn up in these places, ever hear about that warehouse in South Dakota? Yeesh."

Known locations

 * Former Montana
 * Montana bunker (Mentioned in Fallout 3)
 * Former South Dakota
 * Mount Rushmore (Mentioned in Fallout 4)

Appearances
The Northern Commonwealth is mentioned only in Fallout 3, and Fallout: New Vegas and its add-on Honest Hearts. South Dakota's Mount Rushmore appears in the Atomic Command game in Fallout 4.