W.D. Gann, born William Delbert Gann in 1878, was an American financial trader and market theorist renowned for his unique and controversial methods of predicting price movements in the stock and commodities markets. He developed a range of tools and techniques based on mathematics, geometry, astrology, and ancient philosophy, which are collectively known as "Gann Theory."

Gann is best known for his use of geometric angles, time cycles, and price movements to forecast future market trends. His Square of Nine, Gann Angles, and Gann Fan are some of the popular tools he devised, which traders still use today. One of his core beliefs was that price and time are interconnected, and he sought to understand the natural cycles that govern both.

He gained a reputation for making remarkably accurate market predictions. For example, he reportedly forecasted key market events, such as the 1929 stock market crash. Despite the skepticism surrounding his methods, many traders believe his work has value when applied to market timing and technical analysis.

Though Gann’s methods can seem esoteric, his legacy endures through the tools and techniques he pioneered, and they remain a subject of study among market technicians and traders.