Columbia subject headings:
Harvard subject headings:
Monarchy--Europe--History--19th century--Congresses.
Monarchy--Europe--History--20th century--Congresses.
Monarchy--Europe--History--21st century--Congresses.
Princeton subject headings:
Monarchy--Europe--History--19th century.
Monarchy--Europe--History--20th century.
Monarchy--Europe--History--21st century.