MLA

The Royal Duel [Graphic]. Pub. May 27, 1789 by S.W. Fores, No. 3 Piccadilly, 0AD. https://collections.library.yale.edu/catalog/10731119.

APA, 6th edition

The royal duel [graphic]. (0 C.E.). [Etchings England London 1789, Watermarks (Paper)]. Pub. May 27, 1789 by S.W. Fores, No. 3 Piccadilly. https://collections.library.yale.edu/catalog/10731119.