Sonata form is the most widely used form in symphonic and chamber music from the classical period to the twentieth century. It is commonly used in the first movement of a three or four movement composition. Because of this, sonata form is often termed “sonata-allegro” form. This appellation is used because the classical symphony usually begins with a fast (allegro) movement. However, this label is misleading because composers often use sonata form for multiple movements in a single work.
At first blush, sonata form may be viewed strictly as another formal model. However, deeper examination reveals, as theorist D. F. Tovey asserted, that sonata form encourages a certain manner of expression that is both dramatic and dynamic. Indeed, there is an explicit “psychology” built into “sonata style” compositions. Perhaps this mindset is best exemplified by the neat and tidy symmetry inherent in the structural underpinning of the form.
The classical concerto (a multimovement work for orchestra and soloist) usually follows sonata form during at least the first movement. However, in Mozart piano concertos, this strict model becomes quite freewheeling and elastic. In fact, Mozart stretches the parameters and rules of sonata form so much; his concerti are often analyzed as free form fantasias.
Sonata form ultimately reduces to an earlier model called rounded binary (see Chapter 11). This means that sonata form breaks down to an A, B, A1 formula. However, in sonata form, A is represented by the exposition, B is represented by the development, and A1 is represented by the recapitulation. The only element in sonata form that belies rounded binary is the optional coda used by composers to extend the tonic harmony and intensify the movement's conclusion.
The exposition introduces primary thematic material called the first subject group presented in the home key. After the first subject group is stated, the music undergoes a transition and a formal key change. In major keyed symphonies and chamber works, the tonic key usually modulates to the dominant key. In minor keyed symphonies and chamber works, the tonic key usually modulates to the minor dominant or relative major key. After the modulation, secondary thematic material, called the second subject group, is presented. The exposition then concludes with a diminutive codetta or closing theme in the key of the second subject group.
The development expands the material first introduced in the exposition. This section is the least predictable. Usually, there may be many sub-sections of the development and a great deal of harmonic exploration is employed. This includes multiple key shifts, the fragmentation of themes, and playful motivic sequencing. There may even be the introduction of new thematic material.
The composition is at its most unstable during the development if only because it's constantly in a state of harmonic flux. However, for these same reasons, the development is also exciting and emotionally intense. Toward the end of the development, a retransition prepares the music for a restoration of the tonic key, and with it, the return of the first subject group. Often, a protracted dominant seventh chord signals the end of the development.
The recapitulation reinstates the main thematic material in the tonic or home key. Like the exposition, the opening material (first subject group) is presented. This is then followed by a transition, and the reiteration of the second subject group. However, the transition does not include a modulation, and therefore, the second subject group is also presented in the home key. This is then followed by the codetta, also in the tonic key. After the recap, the piece may simply end or move into a coda (see above).