The International Boundary Line between the United States of America and the Dominion of Canada was adopted under article II of
the Convention of London of 1818, under article II of the Webster-Ashburton Treaty of 1842, and under article I of the treaty of 1846, and was modified by articles I and II of the treaty of 1925. The boundary begins at the eastern shore of Georgia Strait and follows the original astronomic determination of the parallel of 49° of north latitude to a point in Lake of the Woods. The International Boundary from the Gulf of Georgia (Georgia Strait) to the Northwesternmost Point of Lake of the Woods is marked by 959 monuments set on the boundary line. The part of the line which follows the astronomic determination of the forty-ninth parallel is commonly called the 49th parallel land boundary. It is 1270.2 miles in length.
The boundary was originally surveyed and marked between 1857 and 1861. In lesser populated areas it was sparsely monumented, so by 1898 questions as to the adequacy of the then present markings of this boundary began to arise. A complete remapping and monumenting of the boundary lying west of the Rocky Mountains took place from 1903 to 1907. From the Pacific Ocean to the summit of the Rocky Mountains there are 272 monuments.
East of the summit of the Rocky Mountains remapping and monumenting took place between 1908 and 1914. The monuments from the summit to Lake of the Woods total 641. Finally, the treaty of 1908 was adopted, providing for the more complete definition and demarcation of the International Boundary between the United States and Canada from the Atlantic Ocean to the Pacific Ocean.
West of the summit of the Rocky Mountains the boundary line is marked by aluminum-bronze monuments, set in concrete bases, the distance between monuments not exceeding 3 miles, save in very exceptional cases where 4 miles should not be exceeded. The same protocol applies east of the summit of the Rocky Mountains to the North Fork of the Milk River. From there east to Lake of
the Woods one-piece hollow cast-iron monuments, filled with concrete, identical in form with those set in 1872-1875, mark the boundary at distances apart not exceeding 2 miles except in a few exceptional cases, where 2¼ miles should not be exceeded.
Paraphrased from the International Boundary Commission Report - 1937