MLA

Brodsky, Joseph, 1940-1996, Miłosz, Czesław. Letter : Czesław Miłosz, 1976 December 20, Berkeley, California to Joseph Brodsky. Box 10 | Folder 263. 1976 December 20. https://collections.library.yale.edu/catalog/10050406.

APA, 6th edition

Brodsky, Joseph, 1940-1996, & Miłosz, Czesław. (["Box 10 | Folder 263"])(1976 December 20)[Letter : Czesław Miłosz, 1976 December 20, Berkeley, California to Joseph Brodsky]. https://collections.library.yale.edu/catalog/10050406.