The terms sonata form, sonata-allegro and first-movement form all describe the same
process. Sonata form became almost standard for the first movement of a symphony,
especially during the period 1780 to 1900. These movements are often marked ‘allegro’
hence the alternative names.
Many late baroque extended binary forms are similar to sonata form but are distinguished
by a separate development section, the simultaneous return of the first subject group and
the tonic, and a full recapitulation of the second subject group.
Introduction
The introduction is optional or may be reduced to a minimum. If it is extended it is
usually slower than the main section and often focuses on the dominant key. It may or
may not contain material which is later stated in the exposition. The introduction
increases the weight of the movement and also permits the composer to begin the
exposition with a theme that would be too light to start on its own as in Haydn’s
symphony no. 103 ‘Drumroll’. The introduction is usually excluded from the repeat of
the exposition. Occasionally material from the introduction reappears in its original
tempo later in the movement. Often this occurs as late as the coda as in Mozart’s string
quintet K 593, Haydn’s ‘Drumroll’ symphony and Beethoven’s ‘Pathétique’ Sonata.
Exposition
The primary thematic material for the movement is presented in the exposition which
may be divided into several sections.
First subject group
This consists of one or more themes, all of them in the home key, also called the tonic.
So if the piece is in C major all of the music in the first group will be in C major.
Transition
In this section the composer modulates from the key of the first subject to the key of the
second subject. Many classical works, however, move straight from the first subject to
the second subject without any transition.
Second subject group
This consists of one or more themes in a different key from the first group. If the first
group is in a major key the second group will usually be in the dominant, so if the
original key is C major the key of the second group will be G major. If the first group is
in a minor key the second group will usually be in the relative major, so if the the original
key is C minor the second group will be in E flat major. The material of the second
group is often different in rhythm and mood from the first group and is often more lyrical.