The Hagia Sophia in Constantinople/Istanbul was commissioned by Emperor Justinian I and built between 532 and 537 by the architects Anthemios of Tralles and Isidoros of Miletus. It was already considered the eighth wonder of the world in the 6th century. No construction plans have survived, raising the question of how this marvel was designed. Volker Hoffmann and Nikolaos Theocharis have sought an answer to this question through several years of research. They presented their findings for the first time in 2005 at an exhibition in Istanbul with 42 posters. They demonstrate that the architects started with a single design figure (which also serves as a "world model"): a square with an inscribed and circumscribed circle. From this, three additional design figures were developed in a strictly geometric manner, and from them—again, consistently geometrically—both the floor plan and the elevation of the church were derived. In addition to reconstructing the design process, new insights into the architects' constructive thinking, the changes to the building after 558, Greek mathematics, and the interplay with cosmology are shared. It appears that the geometric design also symbolizes the ruling ideology of that time, according to which the God-given secular and spiritual authority of the emperor and the patriarch had to be continually reconciled.