Magic line

The American-Canadian border is often referred to as the 49th parallel as it runs for much of its length along the 49th parallel north. It is the longest border between two countries in the world, stretching 8,891 kilometers (5,525 miles) (including the Alaska-Yukon/British Columbia frontier). The continuous border stretches 6,416 kilometres (3987 miles) between Douglas (British Columbia) and Blaine (Washington) on the Pacific coast to Lubeck (Maine) and Welshpool (New Brunswick).
A Convention between the British and Americans in 1818 settled the US and Canadian boundary for the great western expanses at the 49th parallel. This was largely academic as  neither the United Kingdom nor the United States was immediately sovereign over the territories on its side of the line. Control of vast tracts of prarie and forestland still rested with the local First Nations, including the Métis, Assiniboine, Lakota and Blackfoot. Among these nations, the 49th parallel was nicknamed the ‘Medicine Line‘ because of its seemingly magical ability to prevent U.S. soldiers from crossing it.