APA (7th ed.) Citation

Whitman, W., Berenson, M., Hamer, H., & Lawson, W. H. [Postcard, 18]90 Dec. 9, Camden, New Jersey [to] Mary Whitall Costelloe, London, England.

Chicago Style (17th ed.) Citation

Whitman, Walt, Mary Berenson, Harold Hamer, and W. H. Lawson. [Postcard, 18]90 Dec. 9, Camden, New Jersey [to] Mary Whitall Costelloe, London, England.

MLA (8th ed.) Citation

Whitman, Walt, et al. [Postcard, 18]90 Dec. 9, Camden, New Jersey [to] Mary Whitall Costelloe, London, England.