melody to add melodic interest and intensity to the end of the song. Often, the ad lib repeats the previously sung line using variations on phrasing, melodic shape, and/or lyric, but the vocalist may also use entirely new lyrics or a lyric from an earlier section of the song. During an ad lib section, the rhythm may become freer (with the rhythm section following the vocalist), or the rhythm section may stop entirely, giving the vocalist the freedom to use whichever tempo sounds right. During live performances, singers sometimes include ad libs not originally in the song, such as making a reference to the town of the audience or customizing the lyrics to the current events of the era. 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. AABA form 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 intended as a contrast to the A sections that 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 (B 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. 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 B–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. While the form is often