Geometry, Modularity and Proportion in the Extraordinario Libro by Sebastiano Serlio: 50 Portals Between Regola and Licentia

The Extraordinario Libro by Sebastiano Serlio (Extraordinario Libro di Architettura, Per Giovan de Tournes, Lyon, 1551) contains 50 plates that represent models of portals (30 ‘rustic’ and 20 ‘delicate’). These portals are the results of the combination of the five orders’ elements—subject of Book IV (1537)—with rustic ornamentation. The portals theme recurs in Serlio’s Books, and is addressed in Book I (1545) from the geometric point-of-view. The object of the research is to recognize geometric-modular relationships that rule the composition of the 50 portals using graphical analyses. This allows comparing the Serlian models, highlighting the role of geometry as a compositional tool, and identifying modular and dimensional criteria that shape the ‘rustic’ doors and ‘delicate’ doors.