Large forms

Binary and ternary forms fit a dance or a song. The long movements of a symphony, a concerto or a sonata need more room, and they use the same choices, repeat, return, vary or bring in something new, on a larger scale. This topic presents the commonest large forms, sectional ones first, then continuous ones.

Rondo

The rondo was much used for the final movements of classical concertos. It begins with a distinctive theme that returns throughout the movement, the refrain, and between its returns come contrasting sections, the episodes or couplets (Blatter 2007, part 5, art. 4).

In its most basic form the rondo is written A B A C A D A, and so on. The refrain is always in the tonic; the episodes usually are not. Episodes may derive from the refrain or bring new material, but they are never simply repeats of it (Blatter 2007, part 5, art. 4).

Over time variants appeared (Blatter 2007, part 5, art. 4):

Variant	Shape	What changes
two-couplet rondo	A B A C A	only two episodes
variation rondo	A B A′ C A″	the refrain comes back altered each time
sonata rondo	A B A C A B′ A	episode C tends to become a development, and B returns in the tonic
truncated rondo	A B A C B A	B returns after C with no refrain between them

Theme and variations

In theme and variations the first material heard is usually the theme, often chosen from hymns, folk songs or short, familiar pieces, followed by a series of modified versions of it (Blatter 2007, part 5, art. 4).

What a composer changes to make a variation is impossible to predict. The commonest changes are moving the melody to another register, changing the harmony, switching mode, ornamenting the melody, moving from duple to triple, making the accompaniment more complex, and changing timbre, tempo and dynamics (Blatter 2007, part 5, art. 4). A short theme:

Hear the passage

A variation that ornaments the melody:

Hear the passage

A variation that changes the mode:

Hear the passage

In classical variations the first ones stay close to the theme and the later ones stray further, but the theme, its harmony or its rhythm remains recognisable. From the mid-nineteenth century freer variations appeared, closer to a development, in which the theme may not appear at all, or appear only at the end, as if assembled from its remnants (Blatter 2007, part 5, art. 4).

Blatter notes that most jazz pieces are theme and variations. The theme is known to the audience; each full pass over its harmony is a chorus, in which a soloist improvises; and the theme's harmony is kept, enriched by borrowed, substitute and altered chords (Blatter 2007, part 5, art. 4).

Sonata form

The word sonata has two meanings. It is a work in several movements for a solo instrument, or for an ensemble, and it is also the form of one movement, usually the first, of a symphony or sonata. To speak of the form, one says sonata form (Blatter 2007, part 5, art. 4).

It probably evolved from rounded binary, and its classical version settled near the end of the eighteenth century. Seen from afar it looks like a ternary form, with three large sections (Blatter 2007, part 5, art. 4):

Section	What happens	Key
exposition	first theme; first bridge, which modulates; second theme, contrasting; second bridge; closing theme. Usually repeated	starts in the tonic, moves to the dominant (or the relative)
development	the exposition's material is worked out: broken up, recombined, taken into other keys and textures	passes through many keys and returns to a strong dominant
recapitulation	the exposition again, with the same structure	the second theme and everything after it stay in the tonic

The change the recapitulation makes is small and decisive. In the exposition the second theme appeared away from the tonic, creating tension of key; in the recapitulation it returns in the tonic, and the tension resolves (Blatter 2007, part 5, art. 4).

The second theme usually contrasts with the first in character too. If the first is angular, with leaps and dotted rhythms, the second tends to be calmer and to move more by step (Blatter 2007, part 5, art. 4).

Over time the form gained additions: an introduction, often slow, before the exposition; a coda after the recapitulation, which in Beethoven can work as a second development; a codetta closing the exposition; and a retransition at the end of the development, preparing the return (Blatter 2007, part 5, art. 4).

The French overture

The French overture arose in the Baroque, associated with the French court, as a model overture for operas and ballets. It has a slow, stately introduction with dotted rhythms; a faster central section, often polyphonic; and a coda announcing that the performance is about to begin. Blatter tells the traditional explanation: the king would arrive late, and the stately music served for the royal party's entrance (Blatter 2007, part 5, art. 4).

Continuous forms: over a repeating bass

Continuous forms avoid separate phrases with clear cadences. In fact most do have phrases and cadences, but disguised, and the casual listener hears music without breaks (Blatter 2007, part 5, art. 4).

A form that sits between sectional and continuous is the ground bass. A bass line repeats many times, and other lines, often contrapuntal, are laid over it. Each repetition of the bass marks a new section, but the complexity of the upper voices hides the moment of return from almost everyone (Blatter 2007, part 5, art. 4).

When the piece is a lament, or when the words speak of grief, the ground is often a line descending by semitones. A bass of that kind:

Hear the passage

The best-known examples, Blatter recalls, are "Dido's Lament" from Purcell's opera Dido and Aeneas and the Crucifixus from Bach's Mass in B minor (Blatter 2007, part 5, art. 4).

The passacaglia and the chaconne are relatives of the ground bass. Blatter describes them together: the piece is in triple meter; it has either a bass line or a harmonic progression repeated from beginning to end; and over it variations follow, one per repetition. Both names have historically been used for both types, and there are pieces of this kind that bear neither name (Blatter 2007, part 5, art. 4). Bach's Passacaglia in C minor for organ repeats a bass line; the last movement of Brahms's Fourth Symphony repeats a harmonic progression, and has been described both ways.

What comes next

Continuous forms often live on polyphony, and the technique that organises them most is imitation: one voice repeating what another has just said, until it forms a canon, and, at its most elaborate, a fugue.