What is a sonata form exposition?

The exposition is the first main section of a sonata form movement. Its job is to establish the main themes of the movement and to generate tonal, and therefore structural, tension. The exposition can be broadly divided into three and sometimes four sections. These are; the first subject, the transition section, the second subject and the codetta. In regular sonata form, the first and second subjects are the main themes of the movement. The subjects are likely to be contrasting in character; the first is often assertive and strident, with the second being more lyrical. These two themes are joined together by the transition section, while the second subject is often followed by a codetta.

The first and second subjects are also presented in different keys. The first subject is in the tonic of the movement, while the second will be in a subordinate key (a key which is not the tonic). This subordinate key is usually the dominant, in major mode sonatas and the relative major in minor mode ones. The move to a subordinate key is the most important feature of a sonata exposition. By moving away from the home key, the exposition remains structurally incomplete. It therefore requires continuation to the development and recapitulation sections, the latter of which brings the movement’s structure to a conclusion. A good way to think of this last point is to imagine a sonata form movement as a huge question and answer melody. In this analogy, the exposition section becomes a giant question phrase and the recapitulation becomes the answer. The kind of unfinished feeling you can hear in the first “question” phrase, which ends on the dominant, is mimicked on a huge scale in a sonata exposition.