MLA

Backus, Elijah, 1759-1811. Letter : New Haven, Connecticut, to his parents. Box 35, folder GROUP 752, F-2. 1777 Jan. https://collections.library.yale.edu/catalog/17455493.

APA, 6th edition

Backus, Elijah, 1759-1811. (["Box 35, folder GROUP 752, F-2"])(1777 Jan)[Letter : New Haven, Connecticut, to his parents]. https://collections.library.yale.edu/catalog/17455493.