The Introduction section is optional, and 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"). Usually, but not always, the introduction is excluded from the exposition repeat.

Occasionally the material of 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, or Beethoven's Piano Sonata No. 8 ("Pathétique").

For the purpose of illustration we will use the first movement of Mozart's Piano Sonata K545 in C as an example.

This Sonata does not contain an introduction.

Exposition

The themes for the movement are presented in the Exposition. This section is further divided into:

First subject
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. Below is the first subject of our sonata.



Transition – in this section the composer modulates from the key of the first subject to the key of the second. The entire scale passage after the first subject is really part of the transition passage too, but here is the end of it ending in the dominant key of G major.

Second subject
One or more themes in a different key from the first group. If the first subject is in a major key, the second subject will usually be in the dominant. If the first group is in a minor key, the second group will generally be in the relative major. The material of the second subject is often different in rhythm or mood from that of the first group (frequently, it is more lyrical).

Codetta
The purpose of this is to bring the exposition section to a close with a perfect cadence in the same key as the second group. Often the codetta contains a sequence of themes, each of which arrives at a perfect cadence.

The whole of the exposition is usually repeated. Often the last measure or measures of the exposition are slightly different between the repeats, one to point back to the tonic, where the exposition began, and the second to point towards the development.

Development

The development consists of themes from the exposition altered and juxtaposed and may include new material or themes. It can move through many different keys during its course.

The development varies greatly in length from piece to piece, sometimes being relatively short compared to the exposition (e.g. the first movement of Eine kleine Nachtmusik, K 525/I by Mozart) and in other cases quite long and detailed (e.g. the first movement of the "Eroica" Symphony by Beethoven). At the end, the music will turn towards the home key and enter the recapitulation. The transition from the development to the recapitulation is a crucial moment in the work.

The last part of the development section is called the retransition: it prepares for the return of the first subject group in the tonic, most often through a grand prolongation of the dominant seventh. Thus, if the key of the movement is C major, the retransition would most typically stress the dominant seventh chord on G. In addition, the character of the music would signal such a return, often becoming more frenetic (as in the case of the first movement of Beethoven's "Waldstein" Sonata, Op. 53).

Recapitulation

The Recapitulation is an altered repeat of the exposition, and consists of:

First subject
Normally given prominence as the highlight of a recapitulation, it is usually in exactly the same key and form as in the exposition.

Transition
Now altered so that it does not change key, but remains in the piece's home key.

Second subject and codetta
Usually in roughly the same form as in the exposition, but now in the home key, which sometimes involves transformation from major to minor, or vice versa.

Coda

After the final cadence of the recapitulation, the movement may continue with a coda, which will contain material from the movement proper. Codas, when present, vary considerably in length, but, like introductions, are not part of the "argument" of the work. The coda will end, however, with a perfect cadence in the home key. Codas may be quite brief tailpieces, or they may be very long and elaborate. A famous example is the finale of Beethoven's Symphony No. 8 along with the first and fourth movements of the Symphony No. 5; further examples of extended codas from Beethoven include the first movement from the Piano Sonata No. 23 ("Appassionata") and also the third movements from the Piano Sonata No. 14 ("Moonlight") and the Piano Sonata No. 17 ("Tempest").