Grenzsteinroute' , a bicycle route along the border between Germany and the Netherlands. During the route a number of boundary stones ('Grenzsteine' in German) are passed. hence the name. The route starts in Coevorden (NL) then goes to Schoonebeek (NL), Twist (Germany), Zwartemeer (NL), Rütenbrock (Germany), and ends in Ter Apel (NL). Or vice versa of course :)
I'm linking to signs, signs.