may alter melodies, harmonies or time signature at will. The approach to improvisation has developed enormously over the history of the music. In early New Orleans and Dixieland jazz, performers took turns playing the melody, while others improvised countermelodies . By the swing era, big bands were coming to rely more on arranged music: arrangements were either written or learned by ear and memorized, while individual soloists would improvise within these arrangements. Later, in bebop the focus shifted back towards small groups and minimal arrangements; the melody would be stated briefly at the start and end of a piece, but the core of the performance would be the series of improvisations. Later styles such as modal jazz abandoned the strict notion of a chord progression , allowing the individual musicians to improvise even more freely within the context of a given scale or mode. In many forms of jazz a soloist is often supported by a rhythm section that accompanies by playing chords and rhythms that outline the song structure and complement the soloist. [ 12 ] In avant-garde and free jazz idioms, the separation of soloist and band is reduced, and there is license, or even a requirement, for the abandoning of chords, scales, and rhythmic meters. Forms of jazz that are commercially oriented or influenced by popular music have been criticized since at least the emergence of bebop. According to Bruce Johnson, there has always been a "tension between jazz as a commercial music and an art form".