Emerging from the shadows, Langdon and Sophie moved stealthily up the deserted Grand Gallerycorridor toward the emergency exit stairwell.

　　As he moved, Langdon felt like he was trying to assemble a jigsaw puzzle in the dark. The newestaspect of this mystery was a deeply troubling one: The captain of the Judicial Police is trying toframe me for murder"Do you think," he whispered, "that maybe Fache wrote that message on the floor?"Sophie didn't even turn. "Impossible."Langdon wasn't so sure. "He seems pretty intent on making me look guilty. Maybe he thoughtwriting my name on the floor would help his case?""The Fibonacci sequence? The P.S.? All the Da Vinci and goddess symbolism? That had to be mygrandfather."Langdon knew she was right. The symbolism of the clues meshed too perfectly—the pentacle, TheVitruvian Man, Da Vinci, the goddess, and even the Fibonacci sequence. A coherent symbolic set,as iconographers would call it. All inextricably tied.

　　"And his phone call to me this afternoon," Sophie added. "He said he had to tell me something. I'mcertain his message at the Louvre was his final effort to tell me something important, something hethought you could help me understand."Langdon frowned. O, Draconian devil! Oh, lame saint.! He wished he could comprehend themessage, both for Sophie's well-being and for his own. Things had definitely gotten worse since hefirst laid eyes on the cryptic words. His fake leap out the bathroom window was not going to helpLangdon's popularity with Fache one bit. Somehow he doubted the captain of the French policewould see the humor in chasing down and arresting a bar of soap.

　　"The doorway isn't much farther," Sophie said.

　　"Do you think there's a possibility that the numbers in your grandfather's message hold the key tounderstanding the other lines?" Langdon had once worked on a series of Baconian manuscripts thatcontained epigraphical ciphers in which certain lines of code were clues as to how to decipher theother lines.

　　"I've been thinking about the numbers all night. Sums, quotients, products. I don't see anything.

　　Mathematically, they're arranged at random. Cryptographic gibberish.""And yet they're all part of the Fibonacci sequence. That can't be coincidence.""It's not. Using Fibonacci numbers was my grandfather's way of waving another flag at me—likewriting the message in English, or arranging himself like my favorite piece of art, or drawing apentacle on himself. All of it was to catch my attention.""The pentacle has meaning to you?""Yes. I didn't get a chance to tell you, but the pentacle was a special symbol between mygrandfather and me when I was growing up. We used to play Tarot cards for fun, and my indicatorcard always turned out to be from the suit of pentacles. I'm sure he stacked the deck, but pentaclesgot to be our little joke."Langdon felt a chill. They played Tarot? The medieval Italian card game was so replete withhidden heretical symbolism that Langdon had dedicated an entire chapter in his new manuscript tothe Tarot. The game's twenty-two cards bore names like The Female Pope, The Empress, and TheStar. Originally, Tarot had been devised as a secret means to pass along ideologies banned by theChurch. Now, Tarot's mystical qualities were passed on by modern fortune-tellers.

　　The Tarot indicator suit for feminine divinity is pentacles, Langdon thought, realizing that ifSaunière had been stacking his granddaughter's deck for fun, pentacles was an apropos inside joke.

　　They arrived at the emergency stairwell, and Sophie carefully pulled open the door. No alarmsounded. Only the doors to the outside were wired. Sophie led Langdon down a tight set ofswitchback stairs toward the ground level, picking up speed as they went.

　　"Your grandfather," Langdon said, hurrying behind her, "when he told you about the pentacle, didhe mention goddess worship or any resentment of the Catholic Church?"Sophie shook her head. "I was more interested in the mathematics of it—the Divine Proportion,PHI, Fibonacci sequences, that sort of thing."Langdon was surprised. "Your grandfather taught you about the number PHI?""Of course. The Divine Proportion." Her expression turned sheepish. "In fact, he used to joke that Iwas half divine... you know, because of the letters in my name."Langdon considered it a moment and then groaned.

　　s-o-PHI-e.

　　Still descending, Langdon refocused on PHI. He was starting to realize that Saunière's clues wereeven more consistent than he had first imagined.

　　Da Vinci... Fibonacci numbers... the pentacle.

　　Incredibly, all of these things were connected by a single concept so fundamental to art history thatLangdon often spent several class periods on the topic.

　　PHI.

　　He felt himself suddenly reeling back to Harvard, standing in front of his "Symbolism in Art" class,writing his favorite number on the chalkboard.

　　1.618Langdon turned to face his sea of eager students. "Who can tell me what this number is?"A long-legged math major in back raised his hand. "That's the number PHI." He pronounced it fee.

　　"Nice job, Stettner," Langdon said. "Everyone, meet PHI.""Not to be confused with PI," Stettner added, grinning. "As we mathematicians like to say: PHI isone H of a lot cooler than PI!"Langdon laughed, but nobody else seemed to get the joke.

　　Stettner slumped.

　　"This number PHI," Langdon continued, "one-point-six-one-eight, is a very important number inart. Who can tell me why?"Stettner tried to redeem himself. "Because it's so pretty?"Everyone laughed.

　　"Actually," Langdon said, "Stettner's right again. PHI is generally considered the most beautifulnumber in the universe."The laughter abruptly stopped, and Stettner gloated.

　　As Langdon loaded his slide projector, he explained that the number PHI was derived from theFibonacci sequence—a progression famous not only because the sum of adjacent terms equaled thenext term, but because the quotients of adjacent terms possessed the astonishing property ofapproaching the number 1.618—PHI!

　　Despite PHI's seemingly mystical mathematical origins, Langdon explained, the truly mind-boggling aspect of PHI was its role as a fundamental building block in nature. Plants, animals, andeven human beings all possessed dimensional properties that adhered with eerie exactitude to theratio of PHI to 1.

　　"PHI's ubiquity in nature," Langdon said, killing the lights, "clearly exceeds coincidence, and sothe ancients assumed the number PHI must have been preordained by the Creator of the universe.

　　Early scientists heralded one-point-six-one-eight as the Divine Proportion.""Hold on," said a young woman in the front row. "I'm a bio major and I've never seen this DivineProportion in nature.""No?" Langdon grinned. "Ever study the relationship between females and males in a honeybeecommunity?""Sure. The female bees always outnumber the male bees.""Correct. And did you know that if you divide the number of female bees by the number of malebees in any beehive in the world, you always get the same number?""You do?""Yup. PHI."The girl gaped. "NO WAY!""Way!" Langdon fired back, smiling as he projected a slide of a spiral seashell. "Recognize this?""It's a nautilus," the bio major said. "A cephalopod mollusk that pumps gas into its chambered shellto adjust its buoyancy.""Correct. And can you guess what the ratio is of each spiral's diameter to the next?"The girl looked uncertain as she eyed the concentric arcs of the nautilus spiral.

　　Langdon nodded. "PHI. The Divine Proportion. One-point-six-one-eight to one."The girl looked amazed.

　　Langdon advanced to the next slide—a close-up of a sunflower's seed head. "Sunflower seeds growin opposing spirals. Can you guess the ratio of each rotation's diameter to the next?""PHI?" everyone said.

　　"Bingo." Lan............