Within the novel it is claimed that from 1510–1519, Leonardo was the Grand Master of a secret society, the Priory of Sion. In reality this society existed only as a 20th-century hoax, but author Dan Brown used as a source the 1982 pseudohistory book "The Holy Blood and the Holy Grail". The writers of this book had based their research on forged medieval documents that had been created as part of the Priory of Sion fraud. The mix of fact and fiction in the documents made it difficult to discount immediately as a forgery. For example, it was claimed that the Grand Master prior to Leonardo was Botticelli, who had indeed had an association with Leonardo, as they were both students at the Florence workshop of Andrea del Verrocchio.