13th century

The 13th century is the historical period in which the mathematician Leonardo Fibonacci introduced his famous numerical sequence to Europe. This sequence—in which each term is the sum of the two preceding terms—becomes a central element of the mystery in The Da Vinci Code. Jacques Saunière, the murdered curator of the Louvre, scrawls the numbers 13-3-2-21-1-1-8-5 on the floor in his dying moments, a sequence that Sophie Neveu immediately recognizes as a scrambled version of the Fibonacci sequence. The numbers serve multiple purposes: they ensure that the investigation involves cryptographers, thus bringing Sophie into the case, and they also function as a clue to decipher the rest of Saunière's message. The Fibonacci sequence is also the key to the account number for Saunière's safe-deposit box at the Depository Bank of Zurich, which Sophie correctly deduces is 1123581321—the sequence written as a single ten-digit number. The sequence's mathematical properties, particularly the ratio PHI (1.618) derived from the quotients of adjacent terms, are also explored by Robert Langdon in his lectures on the Divine Proportion, linking the sequence to the sacred feminine and the pentacle symbol drawn on Saunière's body. The 13th century thus provides the historical foundation for a mathematical code that drives the plot forward, connecting the worlds of medieval mathematics, Renaissance art, and modern cryptology.