MLA

Adolf, Duke of Schleswig-Holstein-Gottorf, 1526-1586. Letter, 1560 September 10, Schleswig-Holstein, to Elizabeth I, Queen of England. Box. 1560 September 10.. https://collections.library.yale.edu/catalog/16157839.

APA, 6th edition

Adolf, Duke of Schleswig-Holstein-Gottorf, 1526-1586. (["Box"])(1560 September 10.)[Letter, 1560 September 10, Schleswig-Holstein, to Elizabeth I, Queen of England]. https://collections.library.yale.edu/catalog/16157839.