Throughout The Thirteenth Tale, Setterfield raises the motif of the rule of three, a storytelling strategy in literature, and especially fairy tales, in which events, characters, or actions are grouped into threes for the greatest impact.

The rule of three first comes up in Margaret’s early conversations with Vida. When Margaret asks for three verifiable facts that will support Vida’s story, Vida replies, “Ah, the rule of three…The magic number. […] Miss Lea, if you had asked me two questions or four I might have been able to lie, but three…” (49). Vida’s comment shows her understanding, as an author, of the rule of three. Later, in the same conversation, when Vida is about to deliver her third verifiable fact, Margaret thinks, “In the stories with the wizards, it is always with the third wish that everything so dangerously won is disastrously snatched away” (50). By raising the specter of the rule of three early on in the novel, Setterfield establishes a connection to fairy tales and their magic.

When Margaret first visits Angelfield, she discovers Aurelius behind the third door that she opens; when John-the-dig is teaching Vida to trim the topiary, “Three times he let me rest the ladder against the tree before he was satisfied it was safe.