Innerdeutsche Grenze
Until 1952, however, the Inner German border between East Germany and West Germany could be easily crossed in most places. Accordingly, before 1961, most of that east–west flow took place between East and West Germany, with over 3.5 million East Germans emigrating to West Germany before 1961. On August 13, 1961, a barbed-wire barrier, which would become the Berlin Wall separating East and West Berlin, was erected by East Germany.