Borromini's Dome

The space inside is smaller than one of the piers of St Peter's, and yet it never comes to rest. The walls undulate, the columns lean at angles to one another, and wherever you stand the floor plan keeps slipping away from any simple shape.

The dome is where Borromini pulled off his finest trick. The vault is covered with crosses, hexagons and octagons that shrink as they rise, so your eye reads more depth than there actually is. The light falls from windows you can't see from the floor, which makes the dome seem to glow from within.

Everything is white. The order of the Discalced Trinitarians, a group of monks who lived on alms, had no money for marble or gold, and Borromini worked on it largely for nothing. He kept returning to it throughout his life, and said later that his whole idea of architecture was wrapped up in this building.

Beside the church lies a small cloister courtyard, also his work, with rounded corners and a balustrade whose balusters stand upside down in alternation.

We leave the church, cross the intersection and head south until we reach the next church on our left.