Sonata form

Sonata form is a musical form, a way of organising the various themes within a piece. It has been very widely used by classical composers since the 18th century. It was the standard form for the first movement of a symphony, concerto, sonata or other works based on them, like string quartets. For this reason, it is sometimes called first movement form, although this is somewhat of a misnomer, as it has been used in other movements of pieces. It is also sometimes known as compound binary form.

The classical sonata form movement consists of the following sections:

• Exposition - the main themes of the piece are played for the first time. This section can be further divided into:
  • First subject - this is often described at the "first theme", but in fact may consist of more than one melody. However, all of it will be in the home key, so if the piece is in C major, all of the music in the first subject will be in C major.
  • Transition - in this section the composer modulates (that is changes key) from the key of the first subject to the key of the second.
  • Second subject - this will be in a different key to the first subject. If the first subject is in a major key, it will usually be in the dominant, that is to say in a key a perfect fifth higher, so that if the original key is C major, the key of the music in the second subject will be G major. If the first subject is in a minor key, the second subject will generally be in the relative major, so that if the original key is C minor, the second subject will be in E flat major. The material in this section will usually be completely different to that of the first subject, and sometimes will be in marked contrast to it. For example, the first subject may be strident and strongly rhythmic, with the second subject more lyrical.
  • Codetta - a kind of finishing off section, which will bring the exposition section to a close with a perfect cadence in the same key as the second subject. The whole of the exposition may then be repeated.
• Development - this will start in the same key as the exposition ended, and may move through many different keys during its course. It will usually consist of material from the exposition altered and juxtaposed with at least some completely new material. The development usually has a high degree of tonal and rhythmic instability when compared to the other sections. The development section may be quite short, or it may be extremely lengthy. At the end, the music will return to the home key and lead up to the:
• Recapitulation - this is an altered repeat of the exposition, and consists of:
  • First subject - usually in exactly the same form as it appeared in the exposition
  • Transition - now altered so that it does not change key, but remains in the piece's home key
  • Second subject - usually in the same form as in the exposition, but now in the same key as the first subject
  • Coda - another finishing off section, rounding the movement off with a perfect cadence in the home key. Codas may be quite brief tailpieces, or they may be so lengthy as to be almost another development section.

This is the classical sonata form, but it may be varied in a number of ways. The key of the second subject may be something other than the dominant or the relative major - for example, the piano quintet by Johannes Brahms has the first subject in F minor, but the second subject in C sharp minor, a tritone higher. The keys in the recapitulation may also be altered - in the same piece, the second subject in the recapitulation is in F sharp minor, rather than the F minor of the first subject. The recapitulation may also be significantly different from the exposition. In some of Joseph Haydn's pieces, the second subject is exactly the same as the first, the only distinguishing factor being that they are in different keys. So long as the general shape remains the same, the movement can still be said to be in sonata form. The first movements of several symphonies by Gustav Mahler, for example, are described as being in sonata form, although they diverge from the above scheme quite dramatically.

Sonata form shares characteristics with both binary form and ternary form. It terms of key relationships, it is very like binary form, with a first half moving from the home key to the dominant and the second half moving back again (this is why sonata form is sometimes known as compound binary form); in other ways it is very like ternary form, being divided into three sections, the first (exposition) of a particular character, the second (development) in contrast to it, the third section (recapitulation) the same as the first.