exposition
The opening section of a sonata form where the main themes are introduced.
In Depth
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.