San Carlo alle Quattro Fontane (also called San Carlino) is one of the smallest churches in Rome. It was designed by Francesco Borromini in (1638-41) and it is an iconic masterpiece of Baroque architecture. The tight geometric complexity of interlocking ovals and circles creates spaciousness in the small corner church, which stands a stone's throw from the Palazzo Barberini (windows designed by Borromini) and piazza. It is also down the street from rival Gian Lorenzo Bernini's oval Sant'Andrea al Quirinale. The concave-convex facade of San Carlo undulates in a non-classic way. Tall corinthian columns interrupt entablatures.
Idiosyncratic winged hemi-cherubim are used to frame niches of statues, the main one of Saint Charles Borromeo by Antonio Raggi. On the sides are statues of St. John of Matha and St. Felix of Valois, the founders of the Trinitarian Order. The corner fountain is a depiction of recumbent Neptune. The dome of the church has a complex patterns of coffers of crosses, ovals, and hexagons. The floor plan is a heady intersection of ovals.
The church was commissioned by Spanish Trinitarian Order ("Discalced", or "shoeless"), an order dedicated to the freeing Christian slaves. "Quattro Fontane" refers to the four corner fountains at the corners at intersection where the church stands. Two River gods and two goddesses (Juno and Diana): the reclining male figure with the tree is Arno River, and the recumbent male with reeds (no tree) is the Tiber.