Columbia subject headings:
Catholic Church--History--20th century--Congresses.
Pius XI, Pope, 1857-1939--Congresses.
Popes--Biography--Congresses.
Harvard subject headings:
Catholic Church--History--20th century--Congresses.
Pius XI, Pope, 1857-1939--Congresses.
Popes--Biography--Congresses.
Princeton subject headings:
Catholic Church--History--20th century--Congresses.
Pius XI, Pope, 1857-1939--Congresses.
Popes--Biography--Congresses.