Form or Structure in music

On this page:
Phrases
Binary Form
Ternary Form
Composite Ternary Form
Da Capo Arias
Rondo Form
Ritornello Form
Sonata Form
Sonata Rondo Form
Fugue
Theme and Variations
Ground Bass

Phrases
Music is built out of phrases and sentences, in the same way as speech and writing is.   A cadence is the ‘punctuation’ of a musical phrase.  Phrases can be indicated in the music with phrase marks:

Binary form
AB or AABB or ABAB  

Ternary From
ABA

Ternary Form is different from Binary form in that B is a contrasting ‘episode’ with either different material or key.  Some people believe ternary form needs to have contrasting material in the episode, while others disagree.

Composite ternary form (also known as Extended Ternary)
This structure can be found in the Minuet and Trio movements of Classical String Quartets or Symphonies.

Da Capo arias
ABA

These are ternary form pieces for voice, which developed in the Baroque era.  Numerous examples can be found in Handel’s operas and oratorios.  They are also usually in ritornello form.

Rondo Form
ABACADA

A returning theme is interspersed with ‘episodes’.

Ritornello Form
Similar to Rondo form, the principal theme or 'ritornello' returns throughout the piece, separated by episodes. In this way it forms the structural pillars of the movement. The episodes in ritornello form, however, are often based on material from the ritornello theme, and the ritornello theme appears in different keys throughout, usually in this order: tonic; dominant; relative major/minor; subdominant; tonic. 

This form was used for first and last movements of Baroque Concertos.  The full orchestra would play the ritornello theme and the soloist would play the episodes.

The video example is of the first movement of Bach's Violin Concerto in E Major, which is also in Ternary Form.

Sonata Form
Sonata Form developed from binary form.  Some people believe it derives from ternary form since there are three sections:  Exposition, Development, Recapitulation.  But it is clearly related to Rounded Binary Form, since the Development section is not a contrasting episode, rather it is based on the material from the Exposition.

Sonata Rondo Form
This is a combination of Sonata and Rondo Form.  The First Subject is more melodic in this form than in Sonata Form.

Fugue
A fugue is a contrapuntal composition based around one theme, the Subject.

Before we show the form, it is necessary to define a few terms:

The Subject is a single melody which is capable of being treated and developed in a variety of ways.

The Answer  is the subject transposed into the Dominant.  If it is an exact transposition, the Answer is 'real'.  Otherwise it is 'tonal'.  'Tonal' answers occur if the Subject modulates to the Dominant because the Answer must modulate back to the Tonic. 'Tonal' answers also occur is there is a tonic or dominant (or both) note early on in the subject.  The tonic note in this case must be answered by the dominant note and the dominant note must be answered by the tonic in order to keep the Tonic key strong in our minds.

The  Countersubject is the melody the first voice goes onto sing against the Answer when it has finished the Subject.  If the Countersubject is used consistently against each entry of the Subject and Answer, it is called 'Regular'.  If not, it is 'not regular'.  It will need to be modified if the answer is 'tonal'.

A Codetta, which is usually based on material already introduced, is a linking passage and is used to avoid predictability.  

Invertible Counterpoint is achieved when the Subject, Answer and either of the Countersubjects can go in the bass, therefore inverting the parts.  Careful planning is necessary to avoid unprepared 6/4 chords.

Theme and Variations
The first Variations on a theme were composed in the 16th century.  Byrd wrote many.  They tended to be a series of varied harmonisations of the tune and variations of figures in the added parts.  Themes for Variations are usually in binary or ternary form.  Themes can be varied melodically or harmonically.  Melodic Variation means that the harmony is kept intact while either the melody is embellished or new figuration is applied to the harmony.  Later, composers based variations on the principle of the development of motifs.  This is called ‘development variation’. 

Watch the video below to see an analysis of Mozart's variations on 'Ah! que vous dirais-je maman':

Ground Bass
A Ground Bass is a repeating bassline over which there are Variations.  Purcell was the master of this form and wrote many arias in this form for his operas and masques.  The example in the video below is Purcell's 'Music for a While' from Oedipus.  The ground bass disappears for a while in the middle of the aria, and returns with the opening vocal theme to finish the piece.  This is obviously similar to the Da Capo Aria.