Deciphering the Zodiacal Code

The deciphering of the zodiacal code is the pivotal intellectual breakthrough in the investigation, where William of Baskerville, using his knowledge of Arabic cryptography and Aristotelian logic, decodes the secret message left by the murdered translator Venantius of Salvemec. This event transforms a series of baffling clues into a concrete, actionable directive—"Secretum finis Africae manus supra idolum primum et septimum de quatuor"—which directly points to the hidden entrance of the forbidden "finis Africae" and the lost second book of Aristotle's Poetics, the object around which the entire murder plot revolves.

The Discovery of the Coded Message

On the morning of the third day, after the discovery of Venantius's body in the jar of blood, William and Adso of Melk return to the scriptorium to examine the dead translator's desk. They find a single page of parchment that had fallen to the floor, the first half blank and the second half covered with tiny Greek characters. As Adso holds the lamp too close, the flame reveals a second set of markings on the blank side—a series of necromantic-looking symbols that had been written in an invisible ink, likely lemon juice or a substance that reappears when heated. William immediately recognizes this as a deliberate concealment: Venantius had hidden a secret message beneath the Greek text, using a technique known to Arab scholars and described in the works of Roger Bacon, whom William venerates as his master.

The Method of Decryption

William's approach to the code is methodical and draws on multiple intellectual traditions. He first identifies the symbols as a zodiacal alphabet, where each sign of the zodiac and each planet corresponds to a letter of the Latin alphabet. The key, he deduces, lies in the order of the heavens: Earth, Moon, Mercury, Venus, Sun, and then the zodiacal signs in their traditional sequence beginning with Aries. Applying this key, he transforms the string of symbols into the Latin phrase "Secretum finis Africae manus supra idolum primum et septimum de quatuor." The Greek text, which he cannot fully read without his stolen lenses, yields a series of fragmentary, almost delirious phrases about poison, humble persons, rape, and deceit—notes Venantius had taken from the forbidden book itself. William's success is not merely technical; it is a vindication of his philosophical method, which he had earlier demonstrated in the deduction of Brunellus the horse. He tells Adso that the first rule of deciphering a message is to guess what it might mean, then test the hypothesis against the text—a process that mirrors his broader investigative approach of imagining multiple possible orders and discarding those that fail.

The Meaning of the Deciphered Text

The decoded message, "Secretum finis Africae manus supra idolum primum et septimum de quatuor," is a riddle that points directly to the hidden entrance of the "finis Africae," the most secret room in the library. The phrase translates roughly to "The secret of the finis Africae: the hand over the idol works on the first and the seventh of the four." William and Adso initially struggle to interpret it: what is the "idol"? What are the "four" that have a "first" and a "seventh"? The breakthrough comes only later, on the sixth night, when Adso's offhand remark about Salvatore's Latin—"tertius equi" meaning the third letter of the word "equus"—triggers a flash of insight. William realizes that "de quatuor" refers to the word "quatuor" itself, and that the "first and seventh" are its letters Q and R. The "idol" is the mirror in the blind room of the south tower, and the verse above it, "Super thronos viginti quatuor," contains the word "quatuor." Pressing the Q and the R opens the mirror-door to the finis Africae.

Consequences and Significance

The deciphering of the code is the single most important step in William's investigation, transforming a series of seemingly random murders into a coherent pattern centered on a forbidden book. It confirms that Venantius had indeed discovered the secret of the finis Africae and had paid for it with his life. The message also reveals the murderer's method: the poison that killed Venantius, Berengar of Arundel, and later Malachi of Hildesheim was applied to the pages of the book itself, so that anyone who read it with avid curiosity—moistening his fingers to turn the pages—would ingest the lethal substance. The code thus becomes a map not only to the hidden room but to the entire logic of the crimes. William's pride in his achievement is palpable; when Adso naively remarks that they are still where they started, William rebukes him sharply, pointing out that he has succeeded in a few hours in deciphering a code its author believed would remain sealed forever. Yet the message itself remains ambiguous until the final confrontation, and William's triumph is tempered by the realization that his success has come too late to save Severinus of Sankt Wendel or Malachi, and that the book itself will be destroyed before it can be read.