In musical notation, what is the purpose of a double bar line?

A double bar line is primarily used in musical notation to indicate the end of a section or a piece. This is an important visual marker for musicians, as it signals that one musical thought has concluded, and a new one may either begin or the piece itself may be finished. Thus, when a double bar line appears, it provides clarity on the structure of the music, guiding performers in their interpretation and execution.

The other options offer different concepts that do not align with the function of a double bar line. For example, a crescendo would typically be notated with hairpins or expressive markings rather than a double bar line. Similarly, repeats are usually indicated with specific repeat signs, and a new key signature would be shown utilizing a key signature change rather than with a double bar line. Understanding this helps to appreciate how each notation serves a distinct purpose in conveying the composer's intent.