Chiesa di San Carlo alle Quattro Fontane

Rome's most quoted lesson in architecture occupies a corner barely wider than a delivery van's turning circle. San Carlo alle Quattro Fontane, which Romans have always called San Carlino, is the church where Francesco Borromini first worked for himself, and its geometry still draws architects to the Quirinal crossroads with sketchbooks in hand.

Why it matters

The commission gave Borromini his chance. Built for the Order of the Most Holy Trinity, the Trinitarians, whose founders Saint John of Matha and Saint Felix of Valois appear on the facade, the church was designed by Francesco Borromini and raised between 1634 and 1641. Two constraints defined the result, and the church's own presentation dwells on both: the plot was small and irregular, so tight that a popular saying claims the whole building could stand inside one of the four piers beneath Saint Peter's dome, giving Rome the paired proverb that Saint Peter's is beautiful for its size and San Carlino for its smallness. And the budget was poor, so the church was built of plain brick rendered and painted in a single tonal range, its architectural lines compensating for the absence of expensive ornament. Out of those limits Borromini produced an interior that visitors describe as entering a sculpture.

What to notice

Study the plan's arithmetic. Borromini began from a Greek cross, stretched the longitudinal arm, and replaced straight walls with alternating concave and convex curves, so the walls of the upper church seem to breathe. The facade repeats the same idea in elevation: Borromini completed only the lower, undulating order before his death in 1667, and the inscription above the two levels, "In honorem SS. Trinitatis et D. Caroli MDCLXVII", records the year of the upper order's execution. On the facade, statues follow the Trinitarian story: Saint Charles Borromeo between two putti was sculpted by Antonio Raggi in 1675, while the order's founders were carved by Sillano Sillani in 1682.

Beyond the church, the sacristy, crypt and cloister extend Borromini's architectural language; the cloister is a double-height rectangular court: a double-height rectangular court whose corners are opened into convex curves, with pairs of Tuscan columns supporting a serliana run that Borromini modeled on the atria of ancient Roman houses. The original church floor was of terracotta brick and received the present marble paving only in 1898.

Context and history

Borromini arrived at this commission as Francesco Castelli, born in Bissone on 25 September 1599, a mason's son who had worked on the cathedral of Milan as a young man; after San Carlo he took the name Borromini, and the church became the calling card of a highly personal, mathematical architecture informed by his master Carlo Maderno, by Michelangelo and by the theories of Galileo. The Trinitarians' founder-patron, the friar San Giovanni di Bonaventura, left the anecdote the church still tells: when the masons feared to dismantle the scaffolding under the new dome, Borromini stood at the centre of the church while it came down, staking his life on his own geometry.

The site explains the name. The church faces the crossing of two streets at the Quirinal's edge, where four fountains, the Quattro Fontane, mark the corners; the whole complex, church, convent, cloister and crypt, was squeezed onto a sliver of land. Completion stretched past the architect's lifetime, so the building you see spans the work of hands across three decades while remaining unmistakably one idea.