exposition
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The opening section of a sonata form where the main themes are introduced.
The exposition is the opening section of sonata form, where the main musical ideas are presented. It typically introduces two contrasting themes — the first in the home key, the second in a related key (usually the dominant or relative major). A transition passage connects them.
In Classical practice, the exposition is usually repeated to ensure the listener absorbs the themes before they are developed. The relationship between the two themes — their contrast in character, key, and mood — creates the tension that drives the rest of the movement. Mozart's expositions are models of clarity and balance.
In classical practice, the exposition is always repeated — yet many modern conductors skip the repeat, a practice purists consider almost sacrilegious.