Practical and Symbolic Geometry in Borromini’s San Carlo alle Quattro Fontane

Leo Steinberg’s doctoral dissertation of 1960 contained an exposé of the complex geometry of Francesco Borromini’s San Carlo alle Quattro Fontane, and while his scheme has been the starting point for subsequent interpretations, Joseph Connors points out that the majority of drawings relied upon by Steinberg were in fact reworked by Borromini in the 1660s, after the church was built. The geometrical armature of the 1660s plans must therefore be read with caution, measured against the dimensions of the actual building and the geometry discernible in the drawings of the design stage. Whatever the basis of the geometrical reconstruction, something remains unclear, namely, the rationale of the curvature of the lateral chapels. In Practical and Symbolic Geometry in Borromini’s San Carlo alle Quattro Fontane, Michael Hill explores the geometrical rationale of the plan, particularly the role of the biangolo, to unlock its developmental sequence. He also argues for the symbolic importance of the biangolo that provides a cue for a consideration of the plan in terms of an epiphanic representation of the Trinity, a characterization that in turn sheds light on the devotion of San Carlo Borromeo, co-dedicatee of the church, as well as the meaning of the normally ignored altarpieces.