The U.S. and Canada may be as lovey-dovey as two neighbors can get, but according to this charming video history by CGP Grey, both countries agreed to tuck themselves a little bit in, 10 feet back for America, 10 feet back for Canada, creating a corridor of open, surveillable, clear space between them.
This ribbon of emptiness is constantly monitored, regularly gardened (Baby Tree! Be gone!) and persists for 5,500 continuous miles considered the longest deforested straight line in the world protecting the U.S. and Canada from interlopers, or beavers without passports.
Except for one thing it isn't straight.
The engineers who tried to follow the 49th parallel used primitive instruments, and, it appears, twine, and so the border got a wee bit zigzaggy, producing a number of problems, a few of which are delightfully described here ...