Fibonacci (whose real name was Leonardo Pisano) found this sequence as the number of pairs of rabbits months after a single pair begins breeding, assuming that each pair of rabbits produces a pair of offspring when it is two months old.

As , the ratio of successive Fibonacci numbers approaches the limit , known as the "golden ratio". The ancient Greeks regarded as the most aesthetically pleasing proportion for the sides of a rectangle. In this Demonstration the black rectangle has sides proportional to . See how these approach the "golden rectangle" as you increase .

Commercial interests are apparently aware of the appeal of the golden ratio. Credit cards, for which you probably have begun getting offers, have almost exactly this shape.