Kahnweiler as early as 1920, but it was subject to criticism in the 1950s and 1960s, especially by Clement Greenberg. Contemporary views of Cubism are complex, formed to some extent in response to the "Salle 41" Cubists, whose methods were too distinct from those of Picasso and Braque to be considered merely secondary to them. Alternative interpretations of Cubism have therefore developed. Wider views of Cubism include artists who were later associated with the "Salle 41" artists, e.g., Francis Picabia; the brothers Jacques Villon, Raymond Duchamp-Villon and Marcel Duchamp, who beginning in late 1911 formed the core of the Section d'Or (or the Puteaux Group); the sculptors Alexander Archipenko, Joseph Csaky and Ossip Zadkine as well as Jacques Lipchitz and Henri Laurens; and painters such as Louis Marcoussis, Roger de La Fresnaye, František Kupka, Diego Rivera, Léopold Survage, Auguste Herbin, André Lhote, Gino Severini (after 1916), María Blanchard (after 1916) and Georges Valmier (after 1918). More fundamentally, Christopher Green argues that Douglas Cooper's terms were "later undermined by interpretations of the work of Picasso, Braque, Gris and Léger that stress iconographic and ideological questions rather than methods of representation." John Berger identifies the essence of Cubism with the mechanical diagram. "The metaphorical model of Cubism is the diagram: The diagram being a visible symbolic representation of invisible processes, forces, structures. A diagram need not eschew certain aspects of appearance but these too will be treated as signs not as imitations or recreations." Early Cubism: 1909–1914 There was a distinct difference between Kahnweiler's Cubists and the Salon Cubists. Prior to 1914, Picasso, Braque, Gris and Léger (to a lesser extent) gained the support of a single committed art dealer in Paris, Daniel-Henry Kahnweiler, who guaranteed them an annual income for the exclusive right to buy their works. Kahnweiler sold only to a small circle of connoisseurs. His support gave his artists the freedom to experiment in relative privacy. Picasso worked in Montmartre until 1912, while Braque and Gris remained there until after the First World War. Léger was based in Montparnasse. In contrast, the Salon Cubists built their reputation primarily by exhibiting regularly at the Salon d'Automne and the Salon des Indépendants, both major non-academic Salons in Paris. They were inevitably more aware of public response and the need to communicate. Already in 1910 a group began to form which included Metzinger, Gleizes, Delaunay and Léger. They met regularly at Henri