Starting with the scarce historiographical references to Giacomo Della Porta and Domenico Fontana’s involvement in the realisation
of the vaults in the Basilica of St. Petronius, this article reconstructs in detail the sequence of events that set Pope Sixtus V against the
Senate of Bologna. The analysis of the correspondence between Bologna and Rome, in part unpublished, made it possible to establish
the chronology of events: from the choice of Della Porta and Fontana as arbitrators in the bitter diatribe between Terribilia and Carracci regarding the most appropriate height for the Petronian vaults, to the long distance dialogue established by the same architects
with the vestrymen of the Basilica of St. Petronius and, finally, the reports sent from Rome to put an end to the matter.
In addition to emphasising the level of authority outside the Roman borders, the study also highlighted the key role played by
both Fontana and Della Porta during Pope Sixtus’s rule, both of whom were fundamental to the realisation of the Pope’s ambitious
project. A study of the reports produced thanks to this unprecedented collaboration, revealed their evaluations of the two antagonists and, more generally, the complicated Petronian matter, by exploring theoretical reflections on the «true aim of architecture».