Columbia subject headings:
Duchies--Germany--History.
Nobility--Germany--History.
Schleswig-Holstein-Sonderburg-Plön, Dukes of--History.
Schleswig-Holstein-Sonderburg-Plön--History.
Harvard subject headings:
Duchies--Germany--Plön--History--Congresses.
Nobility--Germany--Plön--History--Congresses.
Schleswig-Holstein-Sonderburg-Plön, Dukes of--Congresses.
Schleswig-Holstein-Sonderburg-Plön--History--Congresses.
Princeton subject headings:
Duchies--Germany--Plön--History--Congresses.
Nobility--Germany--Plön--History--Congresses.
Schleswig-Holstein-Sonderburg-Plön, Dukes of--Congresses.
Schleswig-Holstein-Sonderburg-Plön--History--Congresses.