Rule of the mirror

The Rule of the mirror is the secret mechanism that controls the entrance to the finis Africae, the most forbidden chamber of the library. It is a physical and cryptographic puzzle: a large distorting mirror set in a wall of the south tower, whose frame is inscribed with the apocalyptic verse "Super thronos viginti quatuor." The rule dictates that by pressing the first and seventh letters of the word "quatuor" (the Q and the R), the mirror swings open as a door, granting access to the hidden room where Jorge of Burgos has kept the unique copy of Aristotle's lost second book of the Poetics for decades.

Location and Physical Description

The mirror is located in a windowless room of the south tower, a room that also bears the inscription "Super thronos viginti quatuor" on its wall. The mirror itself is taller than a normal man, fixed to the wall by a sturdy oak frame, and its surface is corrugated, grotesquely distorting the images it reflects. When William and Adso first encounter it during their nocturnal exploration of the library, Adso mistakes his own enlarged and misshapen reflection for a devil, crying out in terror. William immediately recognizes the optical trick, noting that the creators of the library surely studied treatises on optics by the Arabs, particularly Alhazen's De aspectibus, which describes how mirrors with specially gauged surfaces can enlarge, invert, or distort images. The mirror is not merely a decorative or frightening device; it is a functional component of the library's defenses, designed to disorient and terrify any intruder who ventures into the labyrinth at night.

The Cryptographic Key

The key to the mirror's secret lies not in the glass itself but in the verse carved above it: "Super thronos viginti quatuor." William, after a flash of insight prompted by Adso's offhand remark about Salvatore's Latin, realizes that the phrase "primum et septimum de quatuor" from Venantius's coded message does not mean "the first and seventh of four" but rather "the first and seventh of the word 'four'"—that is, of the Latin word "quatuor." The word "quatuor" has seven letters: Q, U, A, T, U, O, R. The first letter is Q, and the seventh is R. The letters of the verse are carved or incised in the wall, but those of the word "quatuor" are metal outlines, behind which a hidden mechanism has been walled up. When William presses the Q and then the R, each makes a sharp click, and the entire frame of the mirror shudders. The glass surface snaps back, and the mirror, hinged on its left side, swings open like a door, revealing the entrance to the finis Africae.

Narrative and Thematic Significance

The Rule of the mirror is the final puzzle that William must solve to confront Jorge and uncover the truth behind the murders. It represents the culmination of his deductive method, which relies on reading signs—whether in snow, on parchment, or in the architecture of the library itself. The mirror, a device that distorts reality, is a fitting gateway to a room that contains a book Jorge believes would destroy the very foundations of Christian truth by elevating laughter and mockery to the status of philosophy. The rule also underscores the library's dual nature as a place of both profound order and deliberate deception: the labyrinth is built on mathematical principles, but its defenses—mirrors, vision-inducing herbs, and hidden passages—are designed to exploit human weakness and fear. By solving the riddle of the mirror, William not only gains physical access to the finis Africae but also symbolically penetrates the final layer of Jorge's intellectual and spiritual fortress.