Musical development is one of the three sections of the sonata form, specifically a sonata-allegro movement, which is found in the first movement of a sonata, symphony, or chamber work. Musical development, also known as an episode, is the middle transition section that builds and expands on the thematic elements introduced in the first section, known as the exposition, and leads to the third section, known as the recapitulation. The most artistic freedom can be exercised during the musical development stage. Structure is typically loose, new ideas are introduced, or expository ideas are shaped and expanded, but this is not always the case. A composer can either follow these trends or go in completely different directions, breaking all the rules.
The sonata form, as well as musical development, arose primarily during the Classical period, roughly 1150-1820. This form was used Haydn, Mozart, and Beethoven, though it was not formally referred to as such. Sonata form is not to be confused with sonata composition, which refers to a piece of music with multiple movements, the first of which is usually in sonata form.
In any discussion of musical development, it must be emphasized that, while a template exists, it is not required to be followed, and the so-called rules are frequently broken. One or more thematic structures in a tonic key are established during the exposition, with the possibility of moving on to more themes in contrasting keys. In a single expository section, Schubert was known for using as many as three or four keys.
Thematic elements established in the exposition might be explored musically playing them in different keys or staying in the same key but with a slightly different thematic element to introduce tension. Previous themes may be broken down into fragments and developed in a dynamic manner, or new themes or multiple keys may be introduced. Despite the fact that themes are often changed and broken down, they are still recognizable and linked to the exposition.
During musical development, emotions and passion are heightened. As the composer freely develops the structure set forth in the exposition, new tonal keys may change the mood of the piece. To create dissonance and conflict, multiple keys are sometimes used. The progression is dynamic, fluid, and fraught with drama.
A return to one of the themes introduced in the exposition signals the end of the development section. The drama slows down in the third and final section, the recapitulation, as the original themes are repeated. With the exception of minor variations for interest, the themes are frequently relatively unchanged.