Musical development is one of the three sections of the sonata form, particularly a sonata-allegro movement, a form that usually occurs in the first movement of a whole sonata, symphony, or chamber work. Sometimes called an episode, musical development is the middle transition section that builds and expands upon the thematic elements introduced in the first section, called the exposition, and builds to the third section, the recapitulation. The musical development portion is where the most artistic freedom may be exercised. Typically, but not always, structure is loose, new ideas are introduced, or expository ideas shaped and expanded. A composer may follow these tendencies or may explore completely new directions, breaking all the rules.
The sonata form, including musical development, developed primarily in the Classical period, which occurred around 1150-1820. Haydn, Mozart, and Beethoven used this form, although it was not referred to as such formally. The term "sonata form" should not be confused with sonata composition, which describes a piece of music containing several movements, the first of which is usually in sonata form.
In any discussion of musical development, it must be stressed that although there exists a type of template, it is not required to be followed and frequently the so-called rules are broken. During the exposition, one or more thematic structures are set up in a tonic key, possibly moving on to more themes in contrasting keys. Schubert was known for using as many as three or four keys in a single expository section.
Musical development might explore themes set forth in the exposition by playing them in other keys or staying with the same key but a slightly different thematic element to introduce tension. New themes or multiple keys may be introduced, or previous themes may be broken down into fragments and developed in a dynamic manner. Although themes might be greatly altered and broken down, they still remain somewhat recognizable and connected to the exposition.
Emotions and passion are heightened during musical development. New tonal keys may change the mood of the piece as the composer freely develops the structure set forth in the exposition. Sometimes multiple keys are used to create dissonance and conflict. The development is active, fluid, and fraught with drama.
The end of the development section is signaled by a return to one of the themes introduced in the exposition. During the third and final section, the recapitulation, the drama abates with a restating of the original themes. Frequently, the themes are relatively intact, with the exception of slight variations for interest.