the West every year. East Germany closed the borders between East and West Germany and sealed off the border with West Berlin in 1952; but because of the quadripartite Allied status of the city, the -long sectorial border between East and West Berlin remained open. As there was freedom of movement between West Berlin and West Germany, Easterners could use the city as a transit point to West Germany, usually travelling there by air. To stop this drain of people defecting, the East German government built the Berlin Wall, thus physically closing off West Berlin from East Berlin and East Germany, on 13 August 1961. All Eastern streets, bridges, paths, windows, doors, gates, and sewers opening to West Berlin were systematically sealed off by walls, concrete barriers, barbed wire, and/or bars. The Wall was directed against the Easterners, who by its construction were no longer allowed to leave the East, except with an Eastern permit, not usually granted. Westerners were still granted visas on entering East Berlin. Initially eight street checkpoints were opened, and one checkpoint in the Berlin Friedrichstraße railway station, which was reached by one line of the Western underground (today's U 6), two Western S-Bahn lines, one under and one above ground (approximately today's S 2 and S 3, however, lines changed significantly from 1990 onwards), and transit trains between West Germany and West Berlin started and ended there. The eight street checkpoints were – from North to South along the Wall – on Bornholmer Straße, Chausseestraße, Invalidenstraße, Berlin Friedrichstraße station, Friedrichstraße (Checkpoint Charlie in US military denomination, since this crossing was to their sector), Heinrich-Heine-Straße, Oberbaumbrücke, and Sonnenallee. When the construction of the Wall started after midnight early on 13 August, West Berlin's Governing Mayor Willy Brandt was on a West German federal election campaigning tour in West Germany. Arriving by train in Hanover at he was informed about the Wall and flew to West Berlin's Tempelhof Central Airport. Over the course of the day he protested along with many other West Berliners on Potsdamer Platz and at the Brandenburg Gate. On 14 August, under the pretext that Western demonstrations necessitated it, the East closed the checkpoint at the Brandenburg Gate 'until further notice', a situation that was to last until 22 December 1989, when it was finally reopened. On 26 August 1961 East Germany generally banned West Berliners from entering the Eastern sector. West Germans