This article deals with a proportional analysis of Amiens Cathedral with regard to the medieval tradition of ad triangulum and ad quadratum, whose design principles was commonly used among Medieval masons. It first outlines the notion of ad triangulum and ad quadratum, describes the historical evolution of the notion during the medieval period, and finally interprets the proportional design of the cathedral in analogy with ad triangulum and ad quadratum. It concludes that the proportional design of the cathedral was not quite arbitrary but carefully designed for the systematic as well as symbolic quality of the medieval aesthetic tradition.