MLA

Whitman, Walt, 1819-1892. Whitman, Louisa Van Velsor. Box 2, folder 90. [ca. 1870-76]. https://collections.library.yale.edu/catalog/2053724.

APA, 6th edition

Whitman, Walt, 1819-1892. (["Box 2, folder 90"])([ca. 1870-76])[Whitman, Louisa Van Velsor]. https://collections.library.yale.edu/catalog/2053724.