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Exposition (A)

The exposition of a sonata-form movement is a large, diverse section of music in which the basic material of the movement is presented (or “exposed”). (It is not related to the opening section of a fugue, though they share the same name; see page 128.) The material of a sonata-form exposition consists of the following elements:

To begin, a main theme is presented in the first key, the home or tonic key (see page 31; this key is the key of the piece as a whole — in Mozart’s Symphony in G Minor, the tonic is G minor). This first theme may be a tune, a group of small phrases that sound as though they might grow into a tune, or just a motive or two (see page 25) with a memorable rhythmic character.
After the first theme is firmly established, often with the help of a repetition, there is a change in key, or modulation. The subsection of the exposition that accomplishes this change is called the bridge, or the transition.
The modulation in the bridge is an essential feature (even the essential feature) that gives sonata form its sense of dynamic forward movement. With a little experience, it is not hard to hear the contrast of key and sense the dynamism, for the idea is not to make the crucial modulation sound too smooth. There has to be some tension in the way the new themes, now to be introduced, “sit” in the new key.
Next comes a group of themes or other musical ideas in the new key, called the second group. At least some of these new themes contrast with the first theme in melody, rhythm, dynamics, and so on, as well as in key. Usually one new theme stands out by its melodious quality; this is called the second theme.
The last theme in the second group, the cadence theme, or closing theme, is constructed to make a solid ending prior to a full stop and the big repeat. The very end of the exposition is marked by a loud series of repeated cadences, as though the composer wanted listeners to know exactly where they are in the form. This exposition (A) section is usually repeated.

Freemasonry in the eighteenth century was a high-minded society of intellectuals and aristocrats, promulgating ideas that were often radical. “Enlightened” emperor Joseph II tolerated them, barely. Mozart joined the group and wrote music for their secret meetings; this extraordinary painting shows him seated at the far right. Wien Museum Karlsplatz, Vienna, Austria/ The Bridgeman Art Library.