Figure 4.

Musical structure of the excerpt from the xylophone part of ‘Porgy and Bess’, by G. Gershwin. Each rectangle represents one beat (four sixteenth notes); identical letters and indexes (e.g. a1) denote the repetition of the same musical material; ‘bridge’ indicates a musical section that leads to the following section with different musical material; ‘retro’ means ‘retrograde’ and indicates an inversion of the original pattern; ‘t’ denotes a transposition of the musical material, i.e. a translation of the musical pattern a few bars to the left or to the right on the keyboard of the xylophone; a3’ is a variation on a3, mainly a reorganization of the 3 sub-patterns that constitute this segment