The U.S.-Canada boundary was shaped by a series of treaties that took place between 1783 and 1925.
The first was the Treaty of Paris, which ended the American Revolution, recognized American independence and set the Mississippi River as the western border of the new U.S. — which continued to expand throughout the 19th century.
Years later, the Convention of 1818 officially established the border between the U.S. and British North America — later Canada — at the 49th parallel, from Lake of the Woods, Minn., to the Rocky Mountains. The Oregon Treaty of 1846 extended the border to the Pacific Ocean.
"People sort of sat down and were going back and forth, and lines of latitude became the easiest way to resolve a dispute over who got what," Parmenter says.
Stephen Hornsby, a professor of geography and Canadian studies at the University of Maine, says the choice of the 49th parallel in particular was no coincidence.
The British wanted to preserve the northern part of the continent for the fur trade, while the U.S. wanted as much of the Mississippi basin as possible for agricultural settlement, Hornsby explains. They decided to split the land up based on the river systems, which at the time were the main way of traveling around the continent.
The 49th parallel, Hornsby says, made for an "extraordinarily convenient dividing line." Almost all of the rivers to the north flow out through the St. Lawrence River system or the Hudson Bay, while rivers to the south flow into the Mississippi River and Gulf of Mexico.
"Although it seems like a straight line across the land, when you start to look at the river systems and the larger watersheds, it's basically a dividing line between the watersheds north and south of that line," Hornsby says, adding that "it all makes perfect sense" on a map of North American river systems.
It didn't necessarily make perfect sense in real life, however. Parmenter says lines of latitude didn't take into account geographical formations or the traditional territories of Indigenous people already living there.
"There are Indigenous nations whose territories actually spanned what became the U.S.-Canada border, who come to find out when the survey happens, 'Oh, guess what, this is going right through the middle,' " he says. The Mohawk community of Akwesasne, in northern New York, is one example.