what is inner german border?
It was a border between the German Democratic Republic (East Germany) and the Federal Republic of Germany (West Germany) from 1949 to 1990.
Inner German border
The Inner German border ( or ; initially also ) was the border between the German Democratic Republic (GDR, East Germany) and the Federal Republic of Germany (FRG, West Germany) from 1949 to 1990. 
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Inner German border [SEP] Introduction
The Inner German border ( or ; initially also ) was the border between the German Democratic Republic (GDR, East Germany) and the Federal Republic of Germany (FRG, West Germany) from 1949 to 1990. Not including the similar and physically separate Berlin Wall, the border was long and ran from the Baltic Sea to Czechoslovakia. It was formally established on 1 July 1945 as the boundary between the Western and Soviet occupation zones of former Nazi Germany. On the eastern side, it was made one of the world's most heavily fortified frontiers, defined by a continuous line of high metal fences and walls, barbed wire, alarms, anti-vehicle ditches, watchtowers, automatic booby traps, and minefields.