The exposition module of a sonata movement comes at or near the beginning of a sonata-form movement. Only an introduction (technically, a large prefix) might precede it. It exhibits exposition function, and thus has three jobs: establish the home key, move to and establish a secondary key with a cadence, and lay out the thematic cycle that will serve as the foundation for the development and recapitulation.

There are two main types of sonata expositions: the two-part exposition and the continuous exposition.

The two-part exposition

The two-part exposition is divided into two parts: the first part ends with the arrival of the medial caesura (MC, the major cadence, often accompanied by a musical pause, in the middle of the exposition), and the second part follows the MC and ends with the essential expositional closure (EEC, the major PAC in the secondary key) and any postcadential/closing material that follows it.

A two-part exposition typically exhibits the following thematic cycle (in order):

• Primary theme (P) – home key
• Transition (TR) – ending with the medial caesura (MC)
• Secondary theme (S) – secondary key (V or III), ending with the essential expositional closure (EEC)
• Closing space (C) – secondary key; optional
• Retransition (RT) – recapturing of dominant harmony in home key to prepare repeat of exposition; optional, and only occurs in a first ending

We can abbreviate this cycle:

P TR ' S / C

(The apostrophe stands for the MC; the slash stands for the EEC.)

Each of these terms is defined below.

A sample two-part exposition: W.A. Mozart, Sonata for piano in B-flat major, K. 333, I.

Multiple MCs

Sometimes, a composer sets up a two-part exposition, reaches a satisfactory MC, but the S theme that follows degenerates before it can achieve the EEC. In such a case, the degenerated S theme becomes like a new transition and leads into another MC, followed by an S theme that can reach a satisfactory EEC. Hepokoski and Darcy call such a situation a trimodular block (TMB). We will just call it an exposition with two MCs.

A two-part exposition with two MCs typically exhibits the following thematic cycle (in order):

• Primary theme (P) – home key
• Transition (TR) – ending with the medial caesura (MC)
• "Failed" secondary theme (S?) that does not produce an EEC, but instead leads to a second MC
• "Real" secondary theme (S), ending with the essential expositional closure (EEC)
• Closing space (C) – secondary key; optional
• Retransition (RT) – recapturing of dominant harmony in home key to prepare repeat of exposition; optional, and only occurs in a first ending

We can abbreviate this cycle:

P TR ' S? ' S / C

The two MCs tend to adhere to one of the following patterns (the first being by far the most common; H/D, p. 171):

• I:HC followed by V:HC
• I:HC followed by V:PAC
• V:HC followed by V:PAC
• V:HC followed by V:HC

The continuous exposition

Unlike a two-part exposition, a continuous exposition has no MC followed by a secondary (S) theme. Instead, the transition (TR) module gives way to a "spinning out" of a series of related, fragmented melodic units. This succession of small, "spinning out" modules is called Fortspinnung (Ger., "spinning out"). These fragments are often, but not necessarily, taken from the primary (P) theme. Fortspinnung is often associated with TR in general, but in a continuous exposition, the process gets out of control and fails to produce a satisfactory MC. Instead, the motives continue to "spin out" and maintain a high level of energy right up to the EEC.

The continous exposition follows the thematic cycle:

• Primary theme (P) – home key
• Transition–Fortspinnung (TR=>FS), ending with the essential expositional closure (EEC)
• Closing space (C) – secondary key; optional
• Retransition (RT) – recapturing of dominant harmony in home key to prepare repeat of exposition; optional, and only occurs in a first ending

We can abbreviate it:

P TR=>FS / C

A continous exposition may present no cadences that could be "candidates" for an MC, it may suggest the possibility of an upcoming MC that is evaded, or it may present an MC that fails to produce a satisfactory S theme (and thus is not really an MC). In each case, an EEC is achieved without first arriving at an MC and an S theme.