MLA

Clésinger-Sand, Solange, 1828-1899. Chopin, Frédéric. Box 1, folder 22. undated. https://collections.library.yale.edu/catalog/17296355.

APA, 6th edition

Clésinger-Sand, Solange, 1828-1899. (["Box 1, folder 22"])(undated)[Chopin, Frédéric]. https://collections.library.yale.edu/catalog/17296355.