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The main form of the group embodying the 'sonata principle', the most important principle of musical structure from the Classical period to the 20th century: that material first stated in a complementary key be restated in the home key. Sonata form applies to a single movement, most often part of a multi-movement work such as a sonata, symphony or string quartet; independent movements, e.g. an overture or tone poem, may also be in sonata form. The structure may be considered an expansion of the binary form familiar in Baroque dances, but other genres, including the aria and the concerto, also impinged on its development.
A typical sonata-form movement consists of a two-part tonal structure, articulated in three main sections. The first section ('exposition') divides into a 'first group' in the tonic and, after transitional material, a 'second group' in another key (usually the dominant in major movements, the relative major in minor ones), often with a codetta to round the section off. Both groups may include a number of different themes. In 18th-century music the exposition is almost always directed to be repeated.

The second part of the structure comprises the remaining two sections, the 'development' and 'recapitulation'. The first usually develops material from the exposition in a variety of ways, moving through a number of keys. Compared with the exposition, this section is usually one of considerable tonal instability and of rhythmic and melodic tension. It also prepares the structural climax, the 'double return' to the main theme and to the tonic key which begins the recapitulation. This final section restates the themes of the exposition, usually in the same order; the second group is now heard in the tonic (possibly tonic major if the movement is minor), and there may be temporary excursions to other keys. Before 1780 the second part (development and recapitulation) was usually directed to be repeated. After that date this repetition became increasingly rare; the finale of Beethoven's 'Appassionata' Sonata furnishes a late example.

To the above outline of sonata form may be added a slow introduction and a coda. The primary function of the introduction is to strike a more serious or grander tone and to establish a larger scale of motion than would be possible by the Allegro alone. A coda usually restates the main theme, and most codas include some emphasis on the subdominant, especially if none has occurred in the recapitulation.

The 19th century brought many changes of emphasis: a concentration on contrasting first and second themes rather than on the tonal duality of the exposition; a tendency to avoid exact repetition; and a greatly expanded system of tonal relationships. A sense of strain between structure and content is often manifest, either in an academic approach to the form, as a mould rather than a process, or in the search for new methods of organization, e.g. thematic transformation (Berlioz, Symphonie fantastique; Schumann, Symphony no.4), or a freer approach (e.g. Liszt, Sonata in b Minor; Schumann, Fantasie op.17; Chopin, Ballade in g Minor). Sonata form has nevertheless served for some of the most ambitious and impressive tonal music of the 20th century by composers as different as Strauss and Hindemith, Elgar and Britten, Prokofiev and Shostakovich, and has even shaped movements (e.g. in Schönberg's String Quartets nos.3 and 4) in which tonal centres as such have ceased to exist.

The Grove Concise Dictionary of Music
edited by Stanley Sadie
© Macmillan Press Ltd., London.