What is the significance of the 49th parallel?

After the US acquired the Louisiana Purchase in 1803, a boundary was agreed between the new lands and Rupert's Land (British Lands in North America). The boundary was along the watershed between the Missouri River and Mississippi River basins on one side and the Hudson Bay basin on the other. The British and Americans had concluded a successful meeting in 1817 with the Rush-Bagot Treaty and the scene was set to meet again at the Convention of 1818.