Placement and frequency of the letters do not follow a hard-and-fast rule. Generally they are inserted at places where there is a musically significant change, for example a new theme, or a change in dynamic or instrumentation or the start of a new section – just those places where a conductor might want to restart in rehearsal. In addition, having the letters coincide with musical signposts can help players who are counting rests confirm they are still in the right place, which would not be possible if the marks were placed at numerically regular intervals.