How does a house end up straddling an international border and what?
Here are some pictures from Baarle-Nassau, which kind of lies on the Netherlands/Belgium border Except that the town itself is well within the Netherlands. However, due to the history of land ownership by the Dutch and Belgian crown, certain lands that would otherwise be in the Netherlands have been recognized by ancient treaty to belong to Belgium Now, as you may have noticed, those houses are very old. The problem is the crosses showing where the precise border is are actually fairly new. The people who lived in Baarle-Nassau didnt really care where the border was for 400 years or so. It really only became an issue later on, so its common for a border to go right through a house. So there are a set of rules. The most important one is that your door determines which side of the border your house is on. If your door is in the Netherlands, you pay taxes to the Netherlands government as a Netherlands resident, and if your door is in Belgium, you pay taxes to the Belgian government as a Belgian resident. Since both Belgium and the Netherlands are in the EU, it doesnt really matter if you cross a border, so there are no real border controls. But before the border was marked, no-one could quite be sure if it was. Over the last 400 years, surveying techniques have improved remarkably so that the current border is pretty well marked. Back in the day, it wasnt uncommon for a line of longitude to be marked several hundred meters or so away from where it actually was. A lot of that is because the world isnt a perfect sphere. GPS uses a system where they assume the world is a perfect sphere around someplace in Maryland (where theres an observatory) so the Greenwich observatory, which is by definition at zero longitude, actually lies 300 yards east of where your GPS tells you zero latitude is.