Columbia subject headings:
Authors, Swiss--20th century--Correspondence.
Keller, Gottfried, 1819-1890--Correspondence.
Lawyers--Switzerland--Correspondence.
Petersen, Wilhelm, 1835-1900--Correspondence.
Harvard subject headings:
Authors, Swiss--20th century--Correspondence
Keller, Gottfried, 1819-1890--Correspondence.
Lawyers--Switzerland--Correspondence.
Petersen, Wilhelm, 1835-1900--Correspondence.
Princeton subject headings:
Authors, German--19th century--Correspondence.
Keller, Gottfried, 1819-1890--Correspondence.
Lawyers--Germany--Correspondence.
Petersen, Wilhelm, 1835-1900.