MLA

Williams, William Carlos, 1883-1963. Zorach. Box 26, folder 836. 1939-1940, undated. https://collections.library.yale.edu/catalog/17049572.

APA, 6th edition

Williams, William Carlos, 1883-1963. (["Box 26, folder 836"])(1939-1940, undated)[Zorach]. https://collections.library.yale.edu/catalog/17049572.