 Early big bands typically played 32-bar popular songs or 12-bar blues, which were organized around the traditional jazz two-beat groove; as the big band genre evolved, however, the four-beat swung rhythm became its most substantial change.This was in part a function of the change in the instrumentation of the rhythm section, and the incorporation of the string bass, as well as the substitution of guitar for the banjo.Finally, the musical arrangements that organized an ensemble's playing were based on a "riff approach" which made use of head arrangements.