Columbia subject headings:
Germany. Geheime Staatspolizei--Archives--Catalogs.
Hauptstaatsarchiv Düsseldorf--Catalogs.
Police--Germany--Ruhr (Region)--History--Sources--Bibliography--Catalogs.
Police--Rhine River Valley--History--Sources--Bibliography--Catalogs.
Rhine River Valley--Politics and government--Sources--Bibliography--Catalogs.
Ruhr (Germany : Region)--Politics and government--Sources--Bibliography--Catalogs.
Harvard subject headings:
Germany. Geheime Staatspolizei--Archives--Catalogs.
Hauptstaatsarchiv Düsseldorf--Catalogs.
Police--Germany (West)--Rhineland--History--Sources--Bibliography--Catalogs.
Police--Germany--Ruhr (Region)--History--Sources--Bibliography--Catalogs.
Rhineland (Germany)--Politics and government--Sources--Bibliography--Catalogs.
Ruhr (Germany : Region)--Politics and government--Sources--Bibliography--Catalogs.
Princeton subject headings:
Germany. Geheime Staatspolizei--Archives--Catalogs.
Hauptstaatsarchiv Düsseldorf--Catalogs.
Police--Germany--Rhineland--History--Sources--Bibliography--Catalogs.
Police--Germany--Ruhr (Region)--History--Sources--Bibliography--Catalogs.
Rhineland (Germany)--Politics and government--Sources--Bibliography--Catalogs.
Ruhr (Germany : Region)--Politics and government--Sources--Bibliography--Catalogs.