Columbia subject headings:
Navigation--Portugal--History.
Portugal--Foreign relations--17th century.
Portugal--Foreign relations--18th century.
Portugal--Foreign relations--19th century.
Harvard subject headings:
Navigation--Portugal--History.
Portugal--Foreign relations--17th century.
Portugal--Foreign relations--18th century.
Portugal--Foreign relations--19th century.
Princeton subject headings:
Portugal--Foreign relations--History.
Portugal--Foreign relations--Treaties--History.