Columbia subject headings:
Art, Modern--20th century--Exhibitions.
Art--Private collections--Germany--Exhibitions.
Piepenbrock, Hartwig--Art collections--Exhibitions.
Harvard subject headings:
Art, Modern--20th century--Exhibitions
Art--Private collections--Germany--Exhibitions
Piepenbrock, Hartwig--Art collections--Exhibitions.
Piepenbrock, Maria-Theresia--Art collections--Exhibitions.
Princeton subject headings: