William D. Gann developed the Gann theory in the 1900s. He was a successful trader who believed that stock prices fluctuated in an oblique manner. An asset can move in various directions. He noticed that price changes were related to natural geometric shapes and forecasted future price movements with respect to time. As a result, Gann theory is also known as the "Gann Angle theory."
Gann trading strategy can still be used and, when used correctly, can predict an asset's movement with up to 90% accuracy. W. D. Gann believed in a natural time cycle for the market. Natural geometric shapes and ancient mathematics were the foundations of his theory.
According to Gann theory, the patterns and angles of a market asset can be used to forecast the price's future movements.

Gann Theory, developed by William D. Gann in the early 20th century, is a trading and technical analysis method used to forecast market movements. It combines geometrical, mathematical, and astrological techniques to predict price changes and market trends. Central to Gann Theory are concepts like Gann angles, which are diagonal lines plotted on price charts to identify potential support and resistance levels. Gann believed that time and price share a harmonic relationship, and by analyzing past price movements, one can predict future market behavior.

Gann's tools include the Square of Nine, the Hexagon Chart, and the Circle of 360. These tools help traders identify key price levels and time cycles. Despite its complexity and skepticism among some traders, Gann Theory remains influential and is used by those seeking a deeper understanding of market dynamics.