How did Hagia Sophia combine a rectangular interior effect with a nearly square plan?

The Hagia Sophia achieves its unique interior effect by combining a longitudinal basilica with a centralized building plan. While the building's footprint is nearly square, the interior appears rectangular because of the addition of two great semidomes at the east and west ends. These semidomes effectively prolong the roofline, stretching the visual space along the longitudinal axis.12

This architectural synthesis is further supported by the building's structural design. The main dome is supported by pendentives—spherical triangles that transition the weight of the dome onto four great piers—which allows for a soaring, open central space. On the ground level, the rectangular impression is reinforced by three aisles separated by columns, while the use of curtain walls pierced by windows at the base of the dome creates an illusion that the canopy floats on air, obscuring the structural supports.123