Located at an intersection of four fountains, the San Carlo alle Quatro Fontane maximizes its small site to create a seemingly large space. Borromini incorporates the use of concave and convex bays on the exterior to allude to the space within. Even though the facade is independent of the interior space, much of the morphological elements establish the dynamic relationship between the two.
To expand the space within the church, light enters the lantern to expose the light-toned decorations of the elliptical dome above the cornice transforming into a cross-shaped plan below. The dome itself presents an optical illusion as the geometrical coffering diminishes in size towards the lantern. Many geometries were used to make up the plan of the church. Two equilateral triangles are formed from the intersection of the transverse axes. Within them, multiple circles morph to form the elliptical dome above.
