In 1202, after returning to Italy, Fibonacci documented what he had learned in the "Liber Abaci" ("Book of Abacus"). In doing so, he popularized the use of Hindu-Arabic numerals in Europe.

### The Fibonacci Number Sequence

In the "Liber Abaci," Fibonacci described the numerical series now named after him. In the Fibonacci sequence of numbers, after 0 and 1, each number is the sum of the two prior numbers. Hence, the sequence is as follows: 0, 1, 1, 2, 3, 5, 8, 13, 21, 34, 55, 89, 144, 233, 377, 610 and so on, extending to infinity. Each number is approximately 1.618 times greater than the preceding number.

### The Golden Ratio

This figure – 1.618 – is called Phi or the Golden Ratio. The inverse of 1.618 is 0.618. The Golden Ratio mysteriously appears frequently in the natural world, architecture, fine art, and biology. For example, the ratio has been observed in the Parthenon, Leonardo da Vinci's Mona Lisa, sunflowers, rose petals, mollusk shells, tree branches, human faces, ancient Greek vases and even the spiral galaxies of outer space.

### Fibonacci Levels Used in the Financial Markets

The levels used in Fibonacci retracements in the context of trading are not numbers in the sequence; instead, they are derived from mathematical relationships between numbers in the sequence. The basis of the "golden" Fibonacci ratio of 61.8% comes from dividing a number in the Fibonacci series by the number that follows it.

For example, 89/144 = 0.6180. The 38.2% ratio is derived from dividing a number in the Fibonacci series by the number two places to the right. For example: 89/233 = 0.3819. The 23.6% ratio is derived from dividing a number in the Fibonacci series by the number three places to the right. For example: 89/377 = 0.2360.

Fibonacci retracement levels are depicted by taking high and low points on a chart and marking the key Fibonacci ratios of 23.6%, 38.2%, and 61.8% horizontally to produce a grid. These horizontal lines are used to identify possible price reversal points.

The 50% retracement level is normally included in the grid of Fibonacci levels that can be drawn using charting software. While the 50% retracement level is not based on a Fibonacci number, it is widely viewed as an important potential reversal level, notably recognized in Dow Theory and also in the work of W.D. Gann.