Function and Structure
Sonata-allegro form has three main sections – the Exposition, the Development and the Recapitulation. Even though the form composed of three sections, it is considered a two-part form because it is based on two major modulations in tonality.
W.A. Mozart's String Quartet in G, K. 387 Diagram and Recording.
The beginning opens in the main key of the work. Then after the first the theme, the piece modulates to a foreign key, which is typically five steps above the original key. After arriving in this second key through a direct modulation, the work then modulates back to the original key through many different harmonic colors and ideas.
So the two-part form definition stems from the music beginning in one key, then leaving to another and then going back to the original tonal area. However, the musical ideas of a sonata-allegro form movement are divided into three sections.
The first section called the exposition is similar to an introduction in an essay. The exposition introduces the main themes of the work. Typically a sonata-allegro form movement has two main themes. The first theme is in the tonic, or main key of the piece. The second theme is in a distantly related key. In the classical period, the second key is typically in the key five steps away from the tonic key. Beside the tonal differences in the themes, the two themes typically also vary in style. The first theme is usually something declarative and memorable and the second theme is comparatively lyrical and flowing. A key aspect to the exposition section is the repeat at the end. After hearing the first two themes, the material is repeated exactly the same in the same order so listeners get a second chance to hear it again.
The second section of the work is called the Development. Like the body paragraphs of an essay, the development expands on the themes introduced in the exposition. The development section represents the most creative and unstable section of the work. The development section uses material from the first two themes in many different keys as well as in many different styles. The development section sounds somewhat improvised as the composer uses the thematic material from the exposition to invent new ideas.
The development begins in the same key area as the second theme at the end of the exposition. After going through all the inventions and supporting material, the development ends back in the original key.
The last section of sonata-allegro form is called the Recapitulation. This section is a restatement of the two themes from the exposition. This time however, there is no modulation. The first theme is in the tonic, the main key of the work, and the second theme remains in the tonic as well. At the very end of the work occasionally comes a coda, which is a small section to bring the work to a close. The recapitulation functions as a conclusion which restates the major ideas while closing the work.