Which two countries share the longest border?

The Canada–US border runs for about 8,891 km: roughly 6,416 km between Canada and the lower 48 states and about 2,475 km between Canada and Alaska. A long straight stretch in the west follows the 49th parallel of latitude. The border crosses mountains, prairies, forests and the Great Lakes, and passes through the middle of some towns and even buildings, such as the Haskell Free Library and Opera House, which sits on both sides. The International Boundary Commission keeps a 6-metre-wide strip, called the 'vista', cleared of trees along much of the land border. It is often described as the world's longest undefended border.