The major structural development in composition was the creation of Sonata Form, which the majority of music from the classical period is made up of. With the development of the piano, the instrument replaced the harpsichord and much music was written for solo piano in the form of sonatas or concertos. It was almost always used in the first movements of sonatas, concertos and symphonies. Often, in concerto movements, a cadenza will be included, leading to a grand finale of both solo instrument and the orchestra.
In true sonata form, a whole work is typically made up of four separate, smaller pieces, called movements. These usually have a relationship somehow, perhaps being linked in key structure, thematic material or form. The first and last movements will typically be faster movements, whilst the second movement will usually be a slow movement (Adagio) and the the third movement will often be a Minuet or Scherzo. On occasion, a theme and variations movement is present as the third movement.
Movements written in sonata form usually consist of three main sections; the exposition, the development, and the recapitulation. Within these sections, there are a number of additional sections which may link two sections together. In addition, there is often a coda, which includes much of the material from the original themes presented in a climatic ending which typically includes a cadenza of some sort, where the soloist expands on the thematic material of the movement in a somewhat virtuosic way. This in essence ‘shows off’ the instrument’s and performer’s technical capabilities. This is commonplace for concertos in the classical period, and many performers would compose their own cadenzas, demonstrating their unique abilities and specialities.
The exposition is the first section in a movement in sonata form and presents the main thematic material. An easy way to remember ‘exposition’ is to think of this section as ‘exposing’ the composer’s first themes and ideas. The exposition will often have an introduction before it, so be careful to listen for whether the very beginning is actually the exposition or just an introduction. An easy way to tell is if the beginning of the piece is very slow. Typically the first movement of a sonata, concerto or symphony is fast, so if the beginning of the movement is slow, it is likely an introduction.
The exposition itself is divided into two themes, called subjects. The first subject is always in the tonic key and presents the first main idea. At this point there may be a bridge passage to link to the second subject. The second subject is usually in the dominant key and in a contrasting mood or character to the first subject. It is common to have a codetta after the second subject to round off the exposition section. In most recordings or performances, the exposition will usually be repeated.
The development section is easy to distinguish because the thematic material from the exposition is manipulated and varied a great deal. In the development section, a composer will usually take a snippet of the thematic material from both the first and second subjects in the exposition and literally ‘develop’ them. This could be either a melodic or a rhythmic feature (or both) and these will be changed in terms of modulating through a variety of keys and through rhythmic augmentation or diminution and/or melodic interval changes. In essence, the development section sees a departure from the first and second subject themes and throughout the section, there is often a sense of unrest and turbulence.
The recapitulation section presents the exposition once again, with both the first and second subjects presented in the same way as in the exposition. The major difference here however is that the second subject, rather than being presented in the dominant or a relative key, is presented in the tonic key. This is to allow the ending of the movement to finish neatly in the tonic. In some instances, the recapitulation will also present the slow introduction from the beginning. At the end of the movement, the coda will usually show the culmination of all the thematic material, finishing the movement in the tonic key.
Theme and Variations form:
This was a very common and easy form to use in the Classical period. Mozart used it a great deal, particularly in his works for piano. In fact, the tune to Twinkle Twinkle little star actually comes from a sonata by Mozart in which he utilised a theme and variations structure on the main theme. The theme almost always begins fairly simply, allowing for thematic and technical expansion in the variations which follow. A theme and variations movement will normally consist of a theme with at least 5 variations. The theme will be presented and then gradually, each new movement will expand on the thematic material in an increasingly complex way, resulting in the final movement typically being the most musically and technically demanding for the performer. The variations can vary in terms of tonality (key of the piece, perhaps to the related minor or major), tempo (usually increasing or decreasing the speed and momentum of the movement) and character (perhaps presenting contrasts in mood and emotion).
The rondo form is similar in structure to the theme and variations, but instead of deviating from the original thematic section, the theme or refrain (Section A) repeatedly returns in the tonic key between new sections as follows: A, B, A, C, A, D etc. A typical rondo will usually have only a few alternate contrasting episodes, but there is no real limit to how much new material alternating with section A is presented. The contrasting sections B, C, D normally vary in terms of contrasting tonality, timbre, tempo, and character, perhaps utilising the relative major or minor or diminishing or augmenting the rhythmic features. In essence the rondo form consists of various refrains alternating with episodes of contrasting material, always returning to the original material of the refrain in the tonic key.