MLA

Balʹmont, Konstantin Dmitrievich, 1867-1942. [Postcard: 1923 May 28, Saint-Brevin-les-Pins: Konstantin Dmitrievich Balʹmont to D[agmar] Chahovskoy, Saint-Brevin-l'Océan]. Box 3. 1923 May 28. https://collections.library.yale.edu/catalog/10082165.

APA, 6th edition

Balʹmont, Konstantin Dmitrievich, 1867-1942. (["Box 3"])(1923 May 28)[[Postcard: 1923 May 28, Saint-Brevin-les-Pins: Konstantin Dmitrievich Balʹmont to D[agmar] Chahovskoy, Saint-Brevin-l'Océan]]. https://collections.library.yale.edu/catalog/10082165.