In the course of rehearsing a symphony or piece, it is often necessary for the conductor to stop and go back to some point in the middle, in order to master the more difficult passages or sections, or to resolve a challenge that the ensemble is having. Many scores and parts have bar numbers, every five or ten bars, or at the beginning of each page or line. But as pieces and individual movements of works became longer (extending to several hundred bars) as the Romantic era progressed, bar numbers became less practical in rehearsal.