The writers of a correspondence and the participants in a debate are to be considered as joint authors.

Ex.

Schiller, J: Christoph F: v. Briefwechsel zwischen S. und Cotta; herausg. von Vollmar.

— Briefwechsel zw. S. und Goethe. Stuttg., 1829. 6 v. S.

— Briefwechsel zw. S. und W: v. Humboldt. Stuttg., 1830. S.

Cotta. Briefwechsel. See Schiller, J: C. F: v.

Goethe, J: W. v. Briefwechsel. See Schiller, J: C. F: v.

Humboldt, K: W:, Freiherr v. Briefwechsel. See Schiller, J: C. F: v.

Many catalogues adopt the form of heading

Schiller, J: Christoph F: v., and Humboldt, K: W:, Freiherr v. Briefwechsel. Stuttg., 1830. S.