<P> There is a distinction between ad lib as a song section and ad lib as a general term . Ad lib as a general term can be applied to any free interpretation of the musical material . </P> <P> Thirty - two - bar form uses four sections, most often eight measures long each (4 × 8 = 32), two verses or A sections, a contrasting B section (the bridge or "middle - eight") and a return of the verse in one last A section (AABA). The B section is often designed to be a contrast to the A sections which precede and follow it . The B section may be made to contrast by putting it in a new harmony . For example, with the jazz standard "I've Got Rhythm", the A sections are all tonic prolongations based around the I - vi - ii - V chord progression (Bb in the standard key); however, the B section changes key and moves to V / vi, or D7 in the standard key, which then does a circle of fifths movement to G7, C7 and finally F7, setting the listener up for a return to the tonic Bb in the final A section . </P> <P> The "I've Got Rhythm" example also provides contrast because the harmonic rhythm changes in the B section . Whereas the A sections contain a vibrant, exciting feel of two chord changes per bar (e.g., the first two bars are often Bb g minor / c minor F7), the B section consists of two bars of D7, two bars of G7, two bars of C7 and two bars of F7 . In some songs, the "feel" also changes in the B section . For example, the A sections may be in swing feel, and the B section may be in Latin or Afro - Cuban feel . </P> <P> While the form is often described as AABA, this does not mean that the A sections are all exactly the same . The first A section ends by going back to the next A section, and the second A section ends and transitions into the B section . As such, at the minimum, the composer or arranger often modifies the harmony of the end of the different A sections, so that the listener will be guided through the key changes . As well, the composer or arranger may re-harmonize the melody on one or more of the A sections, to provide variety . Note that with a reharmonization, the melody does not usually change; only the chords played by the accompaniment musicians change . </P>

What is a section of a song called