The Treaty of Paris (1783), concluded between the United States and Britain at the end of the American Revolutionary War, stated that the boundary between the U.S.A. and the British possessions to the north would run "...through the [Lake of the Woods] to the most northwesternmost point thereof, and from thence on a due west course to the river Mississippi..." The parties did not suspect that the source of the Mississippi, Lake Itasca, was south of that point. Consequently the Northwest Angle is the result of 18th-century ignorance of geography. In 1818 the error was corrected by having the boundary run due south from the northwest point of the lake to the 49th parallel and then westward along it. When this north-south line was surveyed, it was found to intersect other bays of the lake and therefore cut off a portion of U.S. territory, now known as the Northwest Angle.
The Northwest Angle has only about 100 inhabitants, and the border crossing is unstaffed. Travelers using the single gravel road into the Angle are expected to use the telephone provided to contact Canadian or U.S. customs and make their declarations.