Columbia subject headings:
Harvard subject headings:
Border security--Europe--History--Congresses.
Boundaries--Social aspects--Europe--Congresses.
Boundaries--Social aspects--Europe.
Europe--Boundaries--History--Congresses.
Schengen Agreement--Congresses.
Princeton subject headings:
Border security--Europe--History--Congresses.
Boundaries--Social aspects--Europe.
Europe--Boundaries--History--Congresses.
Schengen Agreement--Congresses.