The Bolognese architect, historian and collector, Carlo Bianconi, secretary of the Academy of Brera from 1778 and author of a famous guide to Milan published in 1783, collected approximately four hundred original architectural drawings and prints of Milanese buildings dating back to between the 14^th and 18^th centuries in ten large books. In the second book he presented the drawings related to the history of the design of the Duomo. This extremely important collection, which today is housed in the Archivio Storico Civico e Biblioteca Trivulziana, was acquired from Antonio Vallardi in 1872 for the civic collection. The text is of historic significance both for the wealth of graphic documentation it contains and for the original prefaces revealing Carlo Bianconi’s knowledge and critical acumen.