Margen
In the Bidasoa river, between Irun in Spain and Hendaye in France, lies an islet barely two hundred metres long: Pheasant Island. Nobody lives there and you can't visit. Yet it is one of Europe's most curious territories: a condominium that Spain and France take turns to run. From 1 February to 31 July, Spain is in charge; from 1 August to 31 January, France.
The arrangement goes way back. In 1659, after decades of war, the two crowns signed the Peace of the Pyrenees on this very island, which was left as neutral, shared ground. It had already hosted exchanges of princesses and meetings between kings: nobody's land, perfect so that nobody had to give way.
More than three and a half centuries later, the handover still happens right on schedule. It may be the world's most polite border: instead of fighting over the island, the two countries lend it to each other.