Church of San Carlo alle Quattro Fontane

The building site was challenging, as the church was located at the corner of an intersection, and had an adjoining cloister on one side that the architect also designed. Cardinal Francesco Barberini commissioned the church in 1634 and it was the architect's first major commission, though due to various financial difficulties and changes of patronage, it was not completed until after the architect's death. As art critic Olivier Bernier wrote, "based... on geometry... in the handling of form, volume and light... his buildings, although they unmistakably belong to their century, often have a startlingly modern look. Constantly playing with rounded forms - concave and convex cylinders are extensively used - Borromini gives his facades amazing animation without ever relying on mere decoration."

This façade of the Church of San Carlo in Rome innovatively employs convex and concave bays to create an undulating effect. On the lower level, the two outer bays are concave, while the center is convex, thus emphasizing the importance of the entrance. Smaller columns frame niches filled with carved scenes and figures, creating a sense of elaborated depth. A cluster of three statues, depicting St. Charles Borromeo, patron saint of the church, and St. John of Matha and St. Felix of Valois who were also part of the Trinitarian order that Borromeo founded, preside over the central portal. Creating a contrasting vertical energy, four tall columns rise to a curved entablature, from which the four upper columns continue to another curved, but sectioned, entablature. A large oval in the center of the upper entablature, held by two angels that are asymmetrically placed, emphasizes the curvilinear flow of the structure. his façade of the Church of San Carlo in Rome innovatively employs convex and concave bays to create an undulating effect. On the lower level, the two outer bays are concave, while the center is convex, thus emphasizing the importance of the entrance. Smaller columns frame niches filled with carved scenes and figures, creating a sense of elaborated depth. A cluster of three statues, depicting St. Charles Borromeo, patron saint of the church, and St. John of Matha and St. Felix of Valois who were also part of the Trinitarian order that Borromeo founded, preside over the central portal. Creating a contrasting vertical energy, four tall columns rise to a curved entablature, from which the four upper columns continue to another curved, but sectioned, entablature. A large oval in the center of the upper entablature, held by two angels that are asymmetrically placed, emphasizes the curvilinear flow of the structure. 

Borromini's interior was equally innovative, as his plan used a complex interweaving of zones: an undulating lower zone, a middle zone in a traditional Greek cross, and an oval dome that appeared to float above the interior. Rather than depicting images, the dome had a complex but symmetrical geometric pattern that combined unequal hexagons, Greek crosses, and circles with octagonal molding. At its base, clear windows let natural sunlight into the building, and the oculus, too, was clear glass, as the light unified the space with a predominantly white interior.

this marble and stone church is in Rome, Italy.