Columbia subject headings:
Harvard subject headings:
Baltic Sea Region--Commerce--History--Sources.
North Sea Region--Commerce--History--Sources.
Shipping--Baltic Sea Region--History--Sources.
Shipping--North Sea Region--History--Sources.
Princeton subject headings:
Baltic Sea Region--Commerce--History--Sources.
North Sea Region--Commerce--History--Sources.
Shipping--Baltic Sea Region--History--Sources.
Shipping--North Sea Region--History--Sources.