Drawing on paper has been the experimental medium for centuries, serving artists from every period and background. Now on view at Denmark’s National Gallery—rare sketches from the museum’s holdings spanning 500 years. Among the many artists included are Albrecht Dürer, Rembrandt van Rijn, Johann Heinrich Füssli, Christen Købke, Georgia O’Keeffe, Pablo Picasso, and Kirsten Justesen. —E.C.