Borromini

San Carlo alle Quattro Fontane (San Carlino)

Borromini devised the complex ground plan of the church from interlocking geometrical configurations, a typical Borromini device for constructing plans. The resulting effect is that the interior lower walls appear to weave in and out, partly alluding to a cross form, partly to a hexagonal form and partly to an oval form; geometrical figures that are all found explicitly in the dome above. The area of the pendentives marks the transition from the lower wall order to the oval opening of the dome. Illuminated by windows hidden from a viewer below, interlocking octagons, crosses and hexagons diminish in size as the dome rises to a lantern with the symbol of the Trinity.