The small eastern German city of Rudolstadt sits on a curve of the river Saale. All through the summer of 1788, the great poet-philosopher-playwright Friedrich Schiller used to stride around this bend, impatient to meet up with the love of his life, his future wife, Charlotte—but also with her sister Caroline. When he couldn’t see them, he sent love letters, often several a day, and these were sometimes addressed not to one sister but both. They would gather on a bridge across the river. They would swim and sing and talk and read. When the girls’ parents were away, they spent time together in their family home. What happened inside is now unknowable. “You have already become so much to my heart,” Schiller wrote, that formal you being potentially either singular or plural.

Three years later, when Schiller and Charlotte were married and living together in the nearby town of Jena, a young poet named Karl Gotthard Graß became a regular visitor at their house. He once wrote Schiller a letter in which he marveled at the lack of jealousy and quarreling between the two women of the household. “I cannot hide my feelings about the love of these two splendid sisters, for each other and for you,” he wrote. “It was often as if [their mother] had only one daughter and you … had two wives.” It was, the painter continued, just like a fairy tale.