In the 20th century, emphasis moved from the study of themes and keys to how harmony changed through the course of a work and the importance of cadences and transitions in establishing a sense of "closeness" and "distance" in a sonata. The work of Heinrich Schenker and his ideas about "foreground", "middleground", and "background" became enormously influential in the teaching of composition and interpretation. Schenker believed that inevitability was the key hallmark of a successful composer, and that, therefore, works in sonata form should demonstrate an inevitable logic.