Classical music is performed in established genres, such as concertos or symphonies, with an emphasis on melody and accompaniment. It is recognized by lengthy pieces that are rich in rhythmic changes and complex chord progressions. Classical music is often performed by large symphony orchestras.
Early rock and roll, usually performed by small combos, consisted mainly of simple melodies and chord progressions played with a heavily accented four-quarter rhythm. Chord structure began moving toward greater complexity by the late1950s.
During the 1960s and 1970s rock underwent many changes. Some groups, such as Pink Floyd, adopted classical styles by incorporating a more symphonic sound through the use of electronic keyboards as well as through the performance of longer pieces filled with rhythmic changes and complex chord progressions.
Some heavy-metal groups enjoy blurring the distinctions between classical music and rock. In February 2008, the Dutch gothic-metal group, Within Temptation, performed their Black Symphony in Rotterdam with the Dutch Metropole Orchestra and the Pa' Dam Choir.