Chapter 11
Chapter Characters:
Robert Langdon
Sophie Neveu
Bezu Fache
Jérome Collet

Chapter Summary:
At the crime scene in the Louvre, Sophie puts the numbers in order from smallest to largest (1-1-2-3-5-8-13-21) and calls the new order a "Fibonacci sequence." This is "'a progression in which each term is equal to the sum of the two preceding terms.'" For example, 1 + 1 = 2, 1 + 2 = 3, 2 + 3 = 5, and so on.

Langdon offers excuses after he gets off the phone, saying there was an accident requiring him to fly home, and asks to visit the restroom. While he is headed there, Sophie leaves. Fache goes to make a phone call, leaving Collet tracking Langdon electronically.

Chapter Themes:
The human world is encoded with meaning.

Chapter 12
Chapter Characters:
Robert Langdon
Sophie Neveu

Chapter Summary:
Sophie meets Langdon in the restroom, where she tells him that he is being tracked with a surveillance bug and that he is at the Louvre's crime scene not as a symbologist but as a suspect. Fache thinks Langdon killed Saunière, largely because there was a fourth line in the message Saunière wrote: "P.S. Find Robert Langdon." Fache photographed the fourth line, then erased it before Langdon arrived.

Chapter Themes:
The human world is encoded with meaning.
The centrality of human relationships, especially male-female relationships.

Chapter 13
Chapter Characters:
Robert Langdon
Sophie Neveu

Chapter Summary:
Sophie tells Langdon more about why Fache suspects him, then tells Langdon she thinks he is innocent. Saunière's message was left for her, she says, not the police, and the numbered code was written to make sure she, as a cryptographer, gets involved in the investigation of Saunière's death. The Vitruvian Man, Sophie's favorite Da Vinci work, is used to catch her attention, and the P.S. in the message is short for Princesse Sophie, Saunière's nickname for her. Sophie then reveals Saunière is her grandfather.

Chapter Themes
The human world is encoded with meaning.
The centrality of human relationships, especially male-female relationships.
The influence of the past upon the present.

Chapter 14
Chapter Characters:
Bezu Fache
Jérome Collet

Chapter Summary:
Fache and Collet discuss Langdon, and they decide there is no chance that Langdon is onto them. A phone call comes in, telling Fache something is wrong with Sophie.

Chapter Themes:
The human world is encoded with meaning.

Chapter 15
Chapter Characters:
Silas, the albino monk

Chapter Summary:
Silas arrives at the Church of Saint-Sulpice, where he thinks of his goal, and knocks on the door.

Chapter Themes:
The power of belief, and the need to believe.

Chapter 16
Chapter Characters:
Robert Langdon
Sophie Neveu

Chapter Summary:
Sophie recalls her painful memories of her grandfather and their shared past. Ten years ago, she had mistakenly seen her grandfather engaged in something she still can hardly believe, and it ended their relationship. When she was four, her parents, brother, and grandmother died in a car accident. She mentally replays her grandfather's final phone message left that afternoon, including the haunting line, "'Please, I must tell you the truth about your family.'"

Sophie quizzes Langdon on why he was to meet Saunière; Langdon claims he does not know. She recalls her grandfather's message, "Princesse Sophie, Find Robert Langdon," and realizes she needs more time with Langdon. Sophie views their height from the ground and decides it is time for Langdon to leave.

Chapter Themes:
The centrality of human relationships, especially male-female relationships.

Chapter 17
Chapter Characters:
Bezu Fache
Jérome Collet

Chapter Summary:
Collet tries to call Sophie, but there is no answer. Fache reveals to Collet that Sophie's department did not send her and that she is Saunière's granddaughter.

The electronic tracking device planted on Langdon begins to move—out the window, onto the street. Collet concludes he has jumped.

Chapter Themes:
The human world is encoded with meaning.
The power of belief, and the need to believe.

Chapter 18
Chapter Characters:
Robert Langdon
Sophie Neveu
Bezu Fache
Jérome Collet

Chapter Summary:
Langdon and Sophie hide in the Grand Gallery as Fache searches for them. Collet radios Fache that he is picking up Langdon's signal outside. Langdon reviews the past minute, in which Sophie took his bug, put it in a piece of soap, broke a window, and tossed the soap out. Sophie and Langdon can leave the Louvre while the police are chasing the transponder.

Chapter Themes:
The human world is encoded with meaning.
The centrality of human relationships, especially male-female relationships.

Chapter 19
Chapter Characters:
Sister Sandrine
Silas, the albino monk

Chapter Summary:
The Church of Saint-Sulpice's history and architecture is discussed, and the different traditions that contribute to its shape.

Sister Sandrine meets Silas and lets him in. He requests to pray alone. Sister Sandrine hides and watches him.

Chapter Themes:
The human world is encoded with meaning.
The power of belief, and the need to believe.
The influence of the past upon the present.

Chapter 20
Chapter Characters:
Robert Langdon
Sophie Neveu

Chapter Summary:
Langdon and Sophie start to sneak out of the Louvre. As they do, they discuss the meaning of the Fibonacci numbers. Langdon is surprised to discover the pentacle was a special symbol between Sophie and her grandfather when they played Tarot cards. Sophie had been interested in the mathematics of the pentacle, and her grandfather had taught her about PHI (the number 1.618), derived from the Fibonacci sequence. Langdon remembers lecturing on PHI at Harvard, telling his class that plants, animals, and humans all have proportional ratios of PHI and that early scientists called PHI the Divine Proportion. Langdon showed his class examples of the Divine Proportion in works of art, then explained that "'the ratios of line segments in a pentacle all equal PHI, making this symbol the ultimate expression of the Divine Proportion. For this reason, the five-pointed star has always been the symbol for beauty and perfection associated with the godess and the sacred feminine.'"

Langdon suddenly gets the clue to Saunière's code—the Fibonacci numbers are out of order and the letters in "O, Draconian devil! / Oh, lame saint!" are an anagram of Leonardo da Vinci and The Mona Lisa.

Chapter Themes:
The human world is encoded with meaning.
The centrality of human relationships, especially male-female relationships.
The power of belief, and the need to believe.