primum et septimum de quatuor

The phrase 'primum et septimum de quatuor' (the first and seventh of the four) is the cryptic key that William of Baskerville deciphers from Venantius of Salvemec's zodiacal code, enabling him to unlock the secret entrance to the finis Africae, the most forbidden chamber of the abbey's library. This riddle, inscribed in the apocalyptic verse 'Super thronos viginti quatuor' above a distorting mirror, directs the user to press the first and seventh letters of the Latin word 'quatuor'—the 'q' and the 'r'—to open the hidden door. Its solution marks the climax of William's investigation, leading directly to the confrontation with Jorge of Burgos and the revelation of the lost second book of Aristotle's Poetics.

Origin and Decipherment

The phrase first appears in Venantius's secret notes, which William deciphers on the third day. Venantius had encoded the message using a zodiacal alphabet, substituting letters with astrological signs. After hours of labor, William translates the Greek and zodiacal text into the Latin sentence: 'Secretum finis Africae manus supra idolum age primum et septimum de quatuor' (The secret of the finis Africae: the hand over the idol works on the first and seventh of the four). The meaning remains opaque until the sixth day, when Adso of Melk's casual remark about Salvatore's mangled Latin—'tertius equi' (the third of the horse) meaning the letter 'u'—triggers William's insight. He realizes that 'de quatuor' refers not to a numerical set of four objects but to the word 'quatuor' itself, and that 'primum et septimum' denote the first and seventh letters of that word.

The Mechanism of the Mirror

The riddle is physically inscribed in the library's south tower, in a room dominated by a distorting mirror. Above the mirror's frame, carved into the stone, is the apocalyptic verse 'Super thronos viginti quatuor' (Upon the twenty-four thrones). The word 'quatuor' within this verse is the key. William, standing on tiptoe, presses the metal outlines of the letters 'q' (the first) and 'r' (the seventh). Each letter clicks, and the entire mirror frame shudders, swinging open on its left side like a door. This mechanism reveals the hidden entrance to the finis Africae, the heptagonal blind room where Jorge of Burgos has secluded himself with the forbidden book.

Narrative Significance

The solution of 'primum et septimum de quatuor' represents the culmination of William's deductive method, which relies on close observation, linguistic analysis, and the interpretation of signs. It also embodies the novel's central theme of the power and danger of hidden knowledge. The riddle's location—above a mirror that distorts reality—underscores the deceptive nature of appearances and the need to look beyond the surface to uncover truth. The phrase itself, a fragment of a larger apocalyptic text, links the physical act of opening the door to the eschatological anxieties that pervade the abbey, as Jorge's defense of the library is framed as a battle against the Antichrist. Ultimately, the key unlocks not only a room but the final confrontation between William's rational inquiry and Jorge's fanatical faith, a confrontation that ends in the destruction of the library by fire.