One important difference comes in regard to duration. Popular songs are usually brief; most of them are under five minutes long. Classical compositions, on the other hand, range from 20-second pieces to works that last several hours. The average symphonic concert work lasts perhaps half an hour, and this requires a change of perspective for those accustomed to listening to popular songs. How does a composer make such a large piece of music hang together? It's a question worth asking of any piece of music, but for classical compositions it's one of critical importance.