Squaring the circle

What gives Hagia Sophia its beauty is the revolutionary way in which the architects created an immense open space. It is a dome built upon two semi-domes. In order to do this, it was necessary to ‘square the circle’, which means to build a circular dome upon the square base. Hagia Sophia was one of the first structures to make use of this technique. Byzantine paintings of six-winged angels called seraphs (Greek: hexapterygon) still line the pendentives.