The four thematic modules that comprise a typical two-part sonata exposition (P, TR, S, and C) create a thematic rotation that is found again in the recapitulation and often referenced in the development.

Primary theme module (P)

The P theme has several functions: (1) establish the home key, (2) present the primary melodic material to begins the thematic cycle, and (3) begin the motion toward the MC and the EEC.

Most often a P theme is harmonically closed, ending with a I:PAC or, less typically, with a I:HC. (In smaller sonatas, P might be a single phrase (antecedent or presentation) that does not end with a cadence.) When a P theme ends with a PAC and is followed by another theme also ending with a PAC, the P module contains a "primary theme group." The themes in a primary theme group are differentiated with subscripts: P1 and P2.

A P theme may exhibit any standard theme type (sentence, period, hybrid, compound), though sentences are more common. Typically, a P theme is relatively "tight-knit" as compared to other thematic modules, containing a more straightforward presentation of tonic and less phrase deviations.

Transition module (TR)

The TR module's principal functional role is to drive toward the MC that marks its end. This is both a harmonic motion and a rhetorical motion, characterized by energy gain. An analysis of a TR module should center around the MC and how the composer approaches it.

Harmonic motion

In a sonata, a transition may be "modulating" or "non-modulating." A non-modulating TR will lead to a I:HC MC, while a modulating TR leads to a V (or III): HC MC.

Melodic/motivic characteristics

We will follow Hepokoski & Darcy's practice of locating the beginning of TR at the start of a phrase. In general, once you hear TR function clearly projected, track back to the beginning of that phrase and label it the beginning of TR. Unlike primary themes, transitions are much "looser" thematically. This module of the sonata is often associated with phrase expansions and compression. More generally, anything that can be associated with continuation function fits transition function, as well: fragmentation, liquidation, acceleration of melodic or harmonic rhythm, etc.

Melodically, we classify transitions as (1) independent, or (2) dissolving.

An independent TR begins with new thematic material. In other words, it is not P-based.

Dissolving TR modules all take some part of the P theme and dissolve, degenerate, or liquidate as the module gains energy and moves toward the MC.

• In the case of a dissolving restatement, P ends and then seems to begin again. This restatement of P dissolves into TR function, and we can subsequently reinterpret the whole theme as a dissolving-restatement type of TR.
• In a dissolving consequent, continuation, or hybrid, the P module will consist of the opening phrase of a theme, and the closing phrase of the theme will begin as usual but degenerate into TR.
• A dissolving P-codetta will introduce post-cadential material to reinforce the cadence at the end of P, and that post-cadential material will dissolve into TR rhetoric.

In the following example from Beethoven's Piano Sonata in G minor, Op. 49, No. 1, a III: HC MC is clearly found in m. 15 and coincides with the structural HC. Backtracking, notice the continuational characteristics at m. 12 which indicate that the transition must begin at m. 9. (Remember, we always begin a transition at the start of a phrase.) Both the primary theme and transition begin with the same presentation, and therefore, this is a modulating, dissolving transition. As a listener, we will likely not realize that the transition is a transition until we reach m. 12, where the continuation begins to project characteristics associated with TR.

Beethoven, Piano Sonata in G minor, Op. 49, No. 1, i

Subordinate theme module (S)

The chief function of S, which is in the subordinate key of the sonata, is to lead to a PAC in that key — the essential expositional closure (EEC). Because of its role in relation to this central harmonic event (and its corresponding cadence in the recapitulation, the ESC), the S module is of immense importance and interest in a sonata-form movement. Hepokoski and Darcy go so far as to say that "what happens in S makes a sonata a sonata" (p. 117). Because the EEC is of such structural importance, its arrival is often delayed and dramatized to a great degree.

Thematically, the S theme is much looser than the P theme. It is typical to find extended continuations and expanded cadential passages, all as a means towards dramatizing the eventual arrival at the EEC.

It is important to note that new melodic material is not a requirement of S. In fact, many sonatas – especially those composed by Haydn – have S themes that resemble the P theme. (These are called mono-thematic sonatas.)

Closing module (C)

The definitive characteristics of C are that it follows the EEC, and that it is not S. C modules can present wholly new thematic material, or they can borrow from P or TR. They cannot, by definition, be S-based, since that would be a continuation of S. (Keep in mind that the EEC must go on to new material, otherwise the S module continues and the EEC has not been reached yet.)

The C module will always be in the secondary key. It is post-cadential, and the harmonic goal of the exposition has already been reached. If C goes somewhere else, it is not C.

Retransition (RT)

A retransition is like a turnaround in pop/rock or blues music. It is a dominant chord or arrival in the home key that prepares the return to the home key at the beginning of the repeat of the exposition. The difference between an RT and a turnaround is that an RT follows a modulation. When the RT follows a secondary key of V, it turns I in the key of V into V into the key of I by repetition, melodic figuration, or the adding of a chordal seventh.