Columbia subject headings:
Library of Congress--History.
National libraries--United States--History--20th century.
Reading rooms--Washington (D.C.)--History--20th century.
Reference services (Libraries)--Washington (D.C.)--History--20th century.
Harvard subject headings:
Library of Congress--History.
National libraries--United States--History--20th century.
Reading rooms--Washington (D.C.)--History--20th century.
Reference services (Libraries)--Washington (D.C.)--History--20th century.
Princeton subject headings:
Library of Congress--History.
National libraries--United States--History--20th century.
Reading rooms--Washington (D.C.)--History--20th century.
Reference services (Libraries)--Washington (D.C.)--History--20th century.