the first section of a movement in sonata form. It is usually constructed according to the following plan:
Introduction of the first theme (or group of themes) in the tonic; Transition or bridge leading to the dominant (if the first theme is in a major key) or to the relative major key (if the first theme is in a minor key); Second theme (usually contrasted to the first) in the new key; Third or closing theme in a new key. The whole exposition is usually repeated.
