In a world where successful portraiture was equated with artistic value, practice was of the essence. During the Dutch Golden Age, each painting was preceded by a series of detailed sketches—art forms in their own right. For rebels Peter Paul Rubens and Rembrandt van Rijn, sketching became an experimentatal tool, a chance to defy convention. In an exhibition that focuses on sketchbook mementos, a window opens on the inner workings of artistic vision. —E.C.