Which line of latitude forms a significant part of the Canada-United States border?

The 49th parallel forms a significant part of the Canada–United States border.

The 49th parallel north is the circle of latitude located 49 degrees north of the Equator. In North America, roughly 2,030 kilometres of the international boundary follow it, extending from the Strait of Georgia near the Pacific coast eastward to the Lake of the Woods between Manitoba and Minnesota.

The arrangement was established by the Anglo-American Convention of 1818 for much of the boundary west of the Great Lakes and extended west to the Pacific by the Oregon Treaty of 1846. Using a latitude line simplified negotiations that otherwise might have required surveying an uncertain watershed across the plains.

The border is not perfectly identical to the astronomical parallel: nineteenth-century surveys produced deviations of up to about 810 metres. The 49th parallel also does not describe the entire Canada–U.S. border; Alaska, the Northwest Angle and several eastern sections lie elsewhere.