This paper aims at discussing the role played by the art historian Antonio Morassi (Gorizia, 1893-Milan, 1976) in the Guardi forgers affair, an intriguing saga which was of interest to merchants, collectors, and connoisseurs of the Twentieth century art market. The main character is performed by Francesco Guardi (1712-1793), one of the most important Venetian painter of vedute, cherished by forgers since the first half of the 19th century. Thanks to a long process of identification, connoisseurs as Fiocco, Goering and Byam Shaw, and Morassi itself, tried to identify some of these forgers and imitators. The paper tries to investigate the multiples roles of Morassi and his idea of threshold between true and fake, through his expertises, letters, notebooks, auctions catalogues and photos (richly annotated with vivid comments in their back), still preserved in his private Photo Archive, now property of the Department of Philosophy and Cultural Heritage (Ca’ Foscari University of Venice).