A jazz orchestra, be it a small ensemble (trio, quartet, quintet, etc.) or a big band (formation from 10 musicians), is divided into sections. There are basically two sections: the rhythm section and the melody section.
In the rhythm section, there are instruments that generate rhythm (although they can also provide harmonic support), such as drums, guitar (or banjo), double bass (or tuba) and piano.
In the melody section, there are the instruments that exclusively produce melody, such as the wind instruments: trumpet, trombone, clarinet, saxophones, etc.
In the large groups of 10 musicians or more, the so-called big bands, the melody section is divided into two smaller sections: reed instrument section (saxophones and clarinets) and brass instruments section, which, when there are enough of them, can still be subdivided again into two more sections, the trumpet section and the trombone section.

Any performance usually begins with the exhibition of the melody of the theme that is being performed; then there are the improvised solos; and finally, they return to the melody of the theme or a variation of the same.
When it comes to a small ensemble, the exhibition of the melody of the theme at the beginning of a performance is usually done without a score, whether it is executed in unison, harmonized or, as in New Orleans-style ensembles, through what is called a collective improvisation (the trumpet creates the melody and the clarinet and the trombone respectively improvise some voices to complement it).

When it comes to a big band, the exhibition of the melody of the theme is done through what is called an arrangement, that is, an orchestration that sets out what each of the instruments will do. This arrangement can be written, that is, fixed to a score that the musicians read during the execution, or it can be conceived and learned by heart, in what is called an oral arrangement.
During the improvised solos, apart from the fundamental and indispensable (rhythmic and harmonic) support received by the soloist from the rhythm section, the soloist can have this support reinforced through a background, arranged or improvised, from the musicians of the melodic section. This latter support can be based on background notes, maintained and harmonized (chord organs), or based on a riff (insistently repeated rhythmic phrase). This support aims to stimulate the soloist’s inspiration. 

Obviously, for large orchestras, all these mechanisms must have a certain rigidity and are subject to order (fixed in the arrangement); on the other hand, in smaller groups, these interactions can be much more flexible and susceptible to more improvisation.


Ricard Gili, Fundació Catalana Jazz Clàssic