The long, straight, generally undefended border between Canada and the United States is at which latitude?

The Treaty of 1818 saw the expansion of both British North America and the US, where the boundary extended westward along the 49th parallel, from the Northwest Angle at Lake of the Woods to the Rocky Mountains. The treaty extinguished British claims to the south of that line up to the Red River Valley. The treaty also extinguished the US claims to land north of that line in the watershed of the Missouri River, which was part of the Louisiana Purchase. Along the 49th parallel, the border vista is theoretically straight, but in practice follows the 19th-century surveyed border markers and varies by several hundred feet in spots.

An 1844 boundary dispute during the Presidency of James K. Polk led to a call for the northern boundary of the US west of the Rockies to be 54°40′N related to the southern boundary of Russia's Alaska Territory. However, Great Britain wanted a border that followed the Columbia River to the Pacific Ocean. The dispute was resolved in the Oregon Treaty of 1846, which established the 49th parallel as the boundary through the Rockies.

A six-meter opening visible from one concrete pillar to the next allows the boundary to be viewed easily in both directions of any crossing.

Residents of both nations who own property adjacent to the border are forbidden to build within the six-meter-wide (almost 20 feet) boundary vista without permission from the International Boundary Commission. They are required to report such construction to their respective governments.