Traditional New Orleans style jazz was based on a two-beat meter and contrapuntal improvisation led by a trumpet or cornet, typically followed by a clarinet and trombone in a call-response pattern. The rhythm section consisted of a sousaphone and drums, and sometimes a banjo. By the early 1920s guitars and pianos sometimes substituted for the banjo and a string bass sometimes substituted for the sousaphone. Use of the string bass opened possibilities for 4/4 instead of 2/4 time at faster tempos, which increased rhythmic freedom. The Chicago style released the soloist from the constraints of contrapuntal improvisation with other front-line instruments, lending greater freedom in creating melodic lines. Louis Armstrong used the additional freedom of the new format with 4/4 time, accenting the second and fourth beats and anticipating the main beats with lead-in notes in his solos to create a sense of rhythmic pulse that happened between the beats as well as on them, i.e. swing.