Columbia subject headings:
Harvard subject headings:
Berlin Wall, Berlin, Germany, 1961-1989
Borderlands--Bulgaria--History.
Bulgaria--Boundaries.
Escapes--Germany (East)--History.
Greece--Boundaries.
Internal security--Bulgaria--History.
Internal security--Germany--Berlin--History.
Refugees--Germany (East)--History.
Turkey--Boundaries.
Princeton subject headings:
Bulgaria--Politics and government--1944-1990.
Cold War--Political aspects--Bulgaria.
Escapes--Germany (East).