Sonata form can be understood as a complex manifestation of a harmonically open, rounded binary form that is also balanced. Due to its popularity and intricacy, Sonata form has developed its own set of terms to help capture its multiple formal components. At the largest level the, the form is as follows:
The exposition is the term given to a sonata form’s first reprise. The term helps capture the idea that the section is responsible for exposing the main thematic material of the work. In general terms, the exposition can be described as containing a main section in the tonic key, a contrasting section in a non-tonic key, a transition that separates those two sections, and the entire exposition usually ends with a suffix(typically of the large variety).
On the whole, the exposition is a relatively stable part of the form. The primary (P) and secondary (S) themes are expected to be highly stable though the transition between them is unstable. Because it’s a suffix, the closing section is expected to be a very stable part of the form.
A sonata form’s development is the work’s large, unstable section. Like other unstable sections (e.g., B in rounded binary form and C in sonata rondo), the development typically avoids complete thematic statements in favor of sequential passages, chromaticism and modulation, and partial thematic statements. Like the name implies, the development may also “develop” previous material, but this is not a requirement as the development may also introduce its own material. It is common for developments to explore multiple key areas through modulation or extended tonicizations. The sequential passages in developments often involve sequential models that are quite long. For example, in the development of the first movement of Mozart’s Piano Sonata in A minor, a four-measure sequential model is used (see Example I below). Sequences make up such a substantial portion of Classic-era sonata developments, that William Caplin suggests focusing on them when determining their overall structure. He thinks of each sequential passage from its model to its eventual half cadence as a core, suggests the possibility of multiple cores (usually only two), and describes the music between the beginning of the development to the first core as the pre-core. Because developments explore non-tonic keys, they typically end with a retransition (either small or large) that helps to prepare the return of the primary theme in the tonic at the start of the recapitulation.
The recapitulation involves the restatement (in the same order) of material from the exposition, but with the necessary adjustments so that the secondary theme and closing sections are now in the tonic. This section is akin to the middle of a rounded binary form where the opening of the first reprise returns. Sonata recapitulations also feature a balanced aspect because they restate the ending of the first reprise at the end of the second reprise, this time transposed to the tonic key. The return of material in sonata form is more consistent than in binary forms that are balanced. In sonata form, you can expect that the entire secondary theme and the closing section will return in the tonic key. In order for this key change to take place, the restatement usually has to be recomposed somewhere between the primary theme and the start of the secondary theme. The changes often take place during the transition, but it can also happen during the primary theme. If the exposition’s transition ended with a half cadence in the original key (e.g., Mozart, Symphony, no. 25, i), then the recapitulation can actually be restated in full without changes and the secondary theme can simply start in the tonic key with no other required changes.
The exposition’s transition between P and S takes one of two forms, either independent or dependent. The distinction concerns the transition’s melodic/motivic material and whether or not it clearly derives from P. If it is clearly derived from P, the transition is dependent and if it does not, it is independent. An independent transition is usually easier to locate because it sounds like something new instead of a continuation of P. Dependent transitions might begin like a restatement of P but after getting started, veer off in another direction and they typically build energy and feel relatively unstable. A dependent transition typically involves the process of becoming because they initially sound like P is ongoing but as it continues, its transitional function emerges without clear delineation between the two. Another type of dependent transition can occur when P’s suffix doesn’t come to a clear end and instead evolves into a transition through the process of becoming. However, becoming is such a common aspect of dependent transitions in sonata form that most analysts don’t bother labeling it as such.
The medial caesura is a term introduced by James Hepokoski and Warren Darcy that refers to a common phenomenon in late 18th-century sonatas where a mid-expositional break occurs between the end of the transition and the beginning of the secondary theme. While Mozart is an exemplary champion of this technique, it is also used by earlier, later, and contemporaneous composers. As Mark Richards explains, a “medial caesura complex” has three stages, (1) the harmonic preparation, (2) the textural gap, and (3) the acceptance by S. The harmonic preparation happens at the end of the transition and is most commonly a half cadence (often followed by a suffix) in the home key or the upcoming key of S. The textural gap is the literal space between the end of the transition and the beginning of S. In many cases, only rests occur during this gap, but just as often the gap is filled with a single voice that helps to bridge the gap between the two sections (caesura fill). The third part of the medial caesura complex, is the start of S. This means that a convincing feeling of starting a new section (S) will confirm the medial caesura occurred. If the textural gap led instead back to material from the transition and it felt as though the secondary theme never really started, then a true medial caesura would not have occurred because the third stage was missing. Both the exposition and recapitulation can contain a medial caesura, though they may be different because the transition is often recomposed in the recapitulation.
Hepokoski and Darcy also introduced the concepts of Essential Expositional Closure (EEC) and Essential Structural Closure (ESC) in connection with sonata form. These are parallel concepts that refer the first satisfactory PAC in S that moves on to non-S material. In the exposition, this moment is called the EEC and in the recapitulation it is called the ESC. In both situations, this moment determines the end of S and therefore the onset of C. The harmonic goal of the exposition is to establish a new key and produce a PAC in that key and the EEC marks that occasion. The music after the EEC, the Closing section (C), was not necessary for reaching this goal and is therefore an auxiliary section of the exposition, a suffix. The same situation occurs in the recapitulation. The harmonic goal of the recapitulation is that the material from the second half of the exposition is restated in the overall tonic key and that a PAC occurs to confirm that key (ESC).
It is common for sonata forms (especially the first movement of symphonic works) to have a large prefix known as an introduction, or slow introduction. It is common for introductions to contain their own musical material not found in the rest of the work (in the 18th century in particular) and for their tempo to be significantly slower than the tempo of the sonata-form proper. In many cases, the distinction between the end of the introduction and the beginning of the sonata form is quite clear, because the tempo changes abruptly when the sonata-form proper begins.
It is also common for sonata forms to contain a large suffix after the end of the second reprise called a coda (“tail”). As is normal for a suffix, codas are a stable aspect of the form, but particularly long codas might contain unstable portions and they may also revisit material from the rest of the work.
The first movement of Mozart’s sonata in A minor (K. 310) is a relatively clear example of a late 18th-century sonata form. As indicated by the repeat signs at mm. 49,50, and 133, the form has two reprises just like a binary form. As is customary of sonata form, this movement is rounded and features a balanced aspect. Determining the location of a sonata-form’s two core sections (P and S) is an efficient approach for starting a formal analysis. The primary theme (P) begins in m. 1 in the key of A minor (i) and the secondary theme (S) begins in m. 23 in the key of C major (III). It’s expected that S will start somewhere around the middle of the first reprise. This first reprise is 49 measures and half of that would be measure 24.5, so S starting in m. 23 means that it’s really rather close to the middle. Expect that S will start in the middle, not the beginning and not the end of the exposition (1st reprise).
Determining the location of the transition between P and S is a more subtle task. The end of the transition is easy to identify, it’s right before S, but its beginning requires a more detailed investigation. In this case, the transition is of the dependent variety so at first it actually just sounds like P is continuing. The transition starts in m. 9 with a repetition of P’s basic idea but starts to change soon after that in m. 12. At that point, a harmonically unstable passage begins as it modulates to the relative key of C major. The tonic of the new key is most clearly established with the elided half cadence at m. 16 which also marks the beginning of the transition’s suffix. Standing on the dominant persists from that moment until the medial caesura at m. 22. This medial caesura would be described as a III:HC MC because it involves a half cadence in the mediant. There is no literal silence at this moment because caesura fill covers the space with three eight notes that lead to the initiation of S at measure 23, there is however a clear gap in texture in m. 22 as the transition finishes and S starts in the following measure. Remember, at this time (this work was composed in 1778), clear medial caesura’s are very common and this movement can be considered a relatively straight-forward instance of one. Notice also that the transition’s suffix is actually implying that the key is C minor, not C major due to the presence of Ebs. The implication of a minor key here adds an aspect of drama to the end of the transition and then consequently an element of surprise when S ends up being in the major mode even though its preparation suggested otherwise. Mozart doesn’t employ this technique very often but it’s actually pretty common in Beethoven’s music.
Remember that the harmonic goal at the end of the exposition is the EEC, the first PAC in S that moves on to non-S material. In this movement, and in many of his works, Mozart seems to be playing a sort of game with the exact location of this all important moment. It appears that Mozart makes a clear attempt at a PAC in m. 35 but he does two things to prevent it from functioning as the EEC. The first is that he withholds the local tonic in the melody even though the trill in the previous measure suggests that the next note would have been C. Instead the melody rests on the down beat and a stream of sixteenth notes start an octave higher. The second issue is that those sixteenth notes in m. 35 seem very strongly related to S’s melodic/motivic content which gives the impression that S is ongoing instead of it being finished. For these reasons, the potential cadence point has been subverted. Mozart then continues this game by setting up another attempt at the cadence in m. 40 but this time, it’s the bass voice that’s omitted and again S-based material continues. The actual EEC only arrives in m. 45 and it elides with the onset of C which lasts until the end of the exposition in m. 49.
The second reprise (mm. 50-133) can easily be understood as containing two large parts after identifying the returning of the first reprise’s opening material at m. 80. This moment marks the end of the development (the equivalent of B in a rounded binary form) and the beginning of the recapitulation (the equivalent of A’ in a rounded binary form). The development itself is the most unstable portion of the work, due to the variety of chromatic harmonies and large sequential component. It starts by presenting the opening of P’s material in the mediant but quickly veers off into harmonic uncertainty with the enharmonic reinterpretation of m. 57’s chord as an augmented sixth chord in the key of E minor which initiates a large-scale, descending fifths sequence when it resolves in m. 58. The sequential model is very long, four measures in this case, and its copies are stated at m. 62 and m. 66. As Caplin points out, sequences often take up substantial portions of sonata-form developments and their sequential models are often 4 or 8 measures in length. The sequence’s last chord (A dominant seventh, m. 69) resolves to D minor in m. 70 and initiates a modulating retransition that leads back to the tonic key of A minor. The key of A minor is confirmed at the half-cadential arrival at m. 74 which again elides with the onset of a suffix and contains another instance of standing on the dominant though this occurrence has more variety in its bass line than the transition’s suffix had in the exposition. The effect of reaching and maintaining the dominant during this passage however, is still quite audible. This development has a clear distinction between the end of the development and the start of the recapitulation due to the medial-caesura effect that occurs in m. 79 (notice also the chromatic line connecting the two parts that functions as caesura fill), but this boundary can be less clear as well and even involve an elision and in some cases P starts over a dominant pedal making it hard to hear as a clear point of initiation.
As is expected, this recapitulation restates most of the material from the exposition and those materials are presented in the same order. In the exposition S and C were in III and the transition prepared that key by modulating and ending with a half cadence in that key. Those specifics will need to change in order for the ESC to be attained in the overall key of A minor. S and C can simply be transposed from C major to A minor (making sure to account for the difference in mode), but the transition will need to be rewritten to accommodate this change. Like it did in the exposition, the transition in the recapitulation also begins in the ninth measure (m. 88). This transition is still dependent upon P, but it is quite different from the exposition’s version. Notice, however, that they start to become the same again at the half cadence that ends the transition and begins its suffix (compare mm. 16-22 and mm. 97-103). This moment in middle of the recapitulation where a direct correspondence occurs between it and the second half of the exposition is called the cruxby Hepokoski and Darcy. This is also the moment where the balanced aspect of the form comes into play. Like it does in a rounded binary form with a balanced aspect, the crux in a sonata form marks the return of the tail portion of first reprise but transposed to the tonic key. In sonata forms in particular, you should expect that all of S and C will be included in the balanced return as is the case in this movement. Though S and C may be repeated in their entirety without modification, sometimes composers decide to make changes during this restatement. In this case, Mozart expands the second theme in m. 126 by delaying the ESC with a few fully diminished seventh chords that lead back to the dominant (m. 128) which delay the ESC until mm. 129. The closing section does not include recomposition but is simply transposed to the overall tonic key of A minor and no coda follows.