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It passes along or through Germany, Austria, Slovakia, Hungary, Croatia, Serbia, Bulgaria, Romania, Ukraine, and Moldova. It often forms the border between countries: between Slovakia and Hungary from Bratislava to Szob, the northern two thirds of the Croatia-Serbia border, part of the Serbia-Romania border, most of the Romania-Bulgaria border. It also flows through a number of cities, three of which are capitals: Vienna (Austria), Bratislava (Slovakia), Budapest (Hungary).

It passes along or through Germany, Austria, Slovakia, Hungary, Croatia, Serbia, Bulgaria, Romania, Ukraine, and Moldova. It often forms the border between countries: between Slovakia and Hungary from Bratislava to Szob, the northern two thirds of the Croatia-Serbia border, part of the Serbia-Romania border, most of the Romania-Bulgaria border. It also flows through a number of cities, three of which are capitals: Vienna (Austria), Bratislava (Slovakia), Budapest (Hungary).