The paper is devoted to a decisive chapter of the construction history of the twentieth century: one of the most important masterpieces of the contemporary steelworks, the Centre Beaubourg, now the Centre National d’art et de culture Georges-Pompidou. Thanks to the availability of previously unpublished documen- tary records and to interviews with some of the protagonists of the design and construction of this building, it is finally possible to retrace the origins and evolution of the most significant detail of the structure of the Centre Beaubourg: the cast steel piece that later came to be known as the “gerberette”