The 49th parallel of north latitude forms the border between Canada and the United States from Manitoba to British Columbia on the Canadian side and from Minnesota to Washington on the U.S. side. It is a result of the Convention of 1818 and the Oregon Boundary Treaty of 1846.
After the United States acquired the Louisiana Purchase in 1803, it was generally agreed that the boundary between it and British Rupert's Land was along the watershed between the Missouri River and Mississippi River basins on one side and the Hudson Bay basin on the other. However, it is difficult to precisely determine the location of a watershed in a region of level plains, such as in central North America. The British and American committees that met after the War of 1812 to resolve boundary disputes recognized there would be much animosity in surveying the watershed boundary, and agreed on a simpler solution in 1818: the 49th parallel. Both sides gained and lost some territory by this convention, but the United States gained more than it lost. This convention established the boundary only between the Lake of the Woods and the Rocky Mountains; west of the Rockies, the convention established joint occupation of the Oregon Country by both parties.
Although the Convention of 1818 settled the boundary from the point of view of the non-Aboriginal powers, neither the United Kingdom nor the United States was immediately sovereign over the territories on its side of the line: effective control still rested with the local nations, mainly the Métis, Assiniboine, Lakota and Blackfoot. Their sovereignty was gradually ceded by conquest and treaty during the several decades that followed.
In 1846 the Oregon Boundary Treaty divided the Oregon Country between British North America and the United States by extending the 49th parallel boundary to the west coast and then through Strait of Juan de Fuca.
See also: Northwest Angle.