Columbia subject headings:
Harvard subject headings:
Baltic Sea Region--Commerce--History.
Germany, Northern--Commerce--History.
North Sea Region--Commerce--History.
Princeton subject headings:
Baltic Sea Region--Commerce--History.
Germany, Northern--Commerce--History.
Merchant marine--Baltic Sea Region--History.
Merchant marine--Germany, Northern--History.
Merchant marine--North Sea Region--History.
North Sea Region--Commerce--History.