Mark used the rare and massive Polaroid 20x24 Land Camera—of which only three originals exist in North America—to render the images’ extraordinary detail. (As with her previous monograph, Twins, Mark collaborated with her husband, filmmaker Martin Bell, on an accompanying documentary film. In Prom, this comes as a revelatory DVD in the book’s sleeve.) She began the project in 2006, as “the era of Polaroid was rapidly coming to an end,” she notes. “I had to act quickly before the film was discontinued.” Over the next four years, Mark and her crew lugged a 6-foot-tall, 240-pound camera around the nation and chose subjects from 13 proms in 10 cities.