APA (7th ed.) Citation

Frenz, A., & Poensgen, M. (1897). Ex Libris Martha Poensgen.

Chicago Style (17th ed.) Citation

Frenz, Alexander, and Martha Poensgen. Ex Libris Martha Poensgen. [Düsseldorf, 1897.

MLA (9th ed.) Citation

Frenz, Alexander, and Martha Poensgen. Ex Libris Martha Poensgen. 1897.

Warning: These citations may not always be 100% accurate.