Borromini, Francesco, originally Francesco Castelli

Swiss-born Italian baroque architect. He was one of the two most important architects (with Bernini, his main rival) in 17th-century Rome. Whereas Bernini designed in a florid, expansive style, his pupil Borromini developed a highly idiosyncratic and austere use of the classical language of architecture. His genius may be seen in the cathedrals of San Carlo alle Quattro Fontane (1637–41), Sant' Ivo della Sapienza (1643–60), and the Oratory of San Filippo Neri (1638–50).

Borromini was renowned for his brilliant and unorthodox treatment of space and light, with designs based on geometric figures rather than on the proportions of the human body.