What the reader will find below is the result of my research on the design of the Great Pyramid of Cheops. It is a purely mathematical study, scientifically rigorous, yet extraordinarily simple. It is not the formulation of any theory. Quite the contrary: it is the mathematical proof of a theory, which until today had not been possible to be proven, developed by a great researcher of the pyramid, John Taylor (1781-1864), regarding the unit of measurement used for its design. The definitive resolution of a mathematical and architectural enigma that has endured for 4,500 years.