This posthumously published essay presents Hans De Wolf's sharp analysis of one of the greatest rivalries in art history: a rivalry between two centuries, embodied by two of their most prominent figures: Courbet for the 19th and Duchamp for the 20th Century. It is a story in which, in a certain sense, one becomes the perfect enemy of the other. This constellation helped Duchamp to forge his own path, creating an artistic response to Courbet's work with equal relevance and monumentality. The character of this confrontation places the analysis in a historical context that is not purely artistic, not purely historical, and not purely aesthetic. For this reason, it is titled Decorum.For the author, Marcel Broodthaers' installation Décor, which seeks to visualize the 19th and 2th centuries, becomes one of the most important and radical turning points in the history of the 20th Century: a turning point as a metaphor for the reunification of two centuries and the two protagonists who dominated them so profoundly.