San Carlo alle Quattro Fontane site features

It stands on a crossroads, and the building bends. At the corner of Via del Quirinale and Via delle Quattro Fontane — where four fountains sit in four corners, one on each — the front of San Carlo does not run flat past the junction. It waves: the lower storey curves out, in, and out again, so the wall seems to be in slow motion, and columns stand clear of it in the bulges. High up, an oval medallion is held by stone angels. The stone is pale and the street is narrow, so you meet it at an angle, close up, never head on.

Inside it is very small and completely white. The whole church is one room you could cross in a few seconds — the reference work puts it at sixty-six feet by thirty-nine — and after the gold of half of Rome the absence of colour is the first thing you notice. Columns ring the wall, and between them the surface swells and recedes in the same wave as outside.

Then the dome. It is an oval, and it is filled with deep interlocking shapes — octagons, crosses and hexagons — locking into one another like a honeycomb that keeps changing its mind. They are cut deep enough to throw real shadow, and they shrink as they climb, so the dome reads as far taller than it can possibly be. At the top is a small oval lantern ringed with a carved inscription, and light comes down through it onto all that white.

Off the church is the cloister, and it is the quietest thing on the Quirinal: two low storeys of arcading on an octagonal plan, with the corners cut off so the walk has no sharp turns — a small courtyard that feels round. Below the church, a crypt repeats its shape under a low vault.

The dome

Deep interlocking coffering of octagons, crosses and hexagons, diminishing in size as it rises. The single reason to come, and it only works from directly beneath.

The serpentine facade

A front that curves in and out at a crossroads corner. Conceived in the mid-1630s, built at the end of Borromini's life, its upper part finished after his death.

The cloister

Turismo Roma's masterpiece of harmony and proportion: two levels, an octagonal plan, no sharp corners.

The crypt

Below the church, following its size and form, under a low pierced vault.

The four fountains

The crossroads the church is named for, with a fountain set into each of the four corners.

The size

Sixty-six feet by thirty-nine. Smaller than it has any right to feel.