Over the course of the seventeenth and eighteenth centuries, developments in mathematics and the study of statics led to a more precise formalization of the ideas of the traditional constructive practices of arches and vaults, and there was a diffusion of studies on the most stable form for these structures: the catenary curve. Robert Hooke, who first articulated that a catenary arch was comparable to an inverted hanging chain, may have advised Wren on how to achieve the crossing dome of St. Paul's Cathedral. Wren's structural system became the standard for large domes well into the 19th century. The ribs in Guarino Guarini's San Lorenzo and Il Sidone were shaped as catenary arches. The idea of a large oculus in a solid dome revealing a second dome originated with him. He also established the oval dome as a reconciliation of the longitudinal plan church favored by the liturgy of the Counter-Reformation and the centralized plan favored by idealists. Because of the imprecision of oval domes in the Rococo period, drums were problematic and the domes instead often rested directly on arches or pendentives.