Columbia subject headings:
Art, Abstract--Europe--Exhibitions.
Art, Abstract--Germany--Exhibitions.
Art, European--20th century--Exhibitions.
Art, German--20th century--Exhibitions.
Piepenbrock, Hartwig--Art collections--Exhibitions.
Harvard subject headings:
Art, Abstract--Europe--Exhibitions.
Art, Abstract--Germany--Exhibitions
Art, European--20th century--Exhibitions
Art, German--20th century--Exhibitions.
Piepenbrock, Hartwig--Art collections--Exhibitions.
Princeton subject headings:
Art, German--20th century--Exhibitions.
Piepenbrock, Hartwig--Art collections--Exhibitions.
Piepenbrock, Maria-Theresia--Art collections--Exhibitions.