Where did the Grammaire latine en figures allégoriques empruntées à l’art militaire, given to the young Gaston d’Orléans in 1615, come from? Who wrote it? What purpose did it serve? Why did this lavishly illuminated ceremonial manuscript fall into such disrepute in the nineteenth century? This paper, embracing a genetic and historiographic approach, aims to offer some tentative answers to those many questions.