Golden Rectangle
In the plot of The Da Vinci Code, they use several works by Leonardo da Vinci and have the characters either solve puzzles or use them as examples of ideas that may be encoded in the picture. Like many artists, Leonardo used the Golden Rectangle while making the paintings.
The Golden Rectangle is that sides are in proportion with the Golden Ratio. The longer side is 1.618 times longer than the shorter side. Both the Golden Rectangle and Ratio are seen as pleasing to the eye and found in many areas of art and culture. An example of this would be the Parthenon in Athens, Greece.
Leonardo used the Golden Rectangle in his drawing of the Vitruvian Man. He has the outlines of a rectangle based on the head, one on the torso, and another over the legs.