Sonata form is the architectural design — exposition, development, recapitulation — that organised much of classical and romantic instrumental music.
Sonata form (sometimes called sonata-allegro form) crystallised in the second half of the 18th century in the works of Haydn, Mozart, and the early Beethoven. It organises a single movement into three sections: an exposition presenting two contrasting themes (typically in tonic and dominant keys), a development that fragments and modulates them through distant harmonies, and a recapitulation that restates both themes in the home key, achieving resolution. The form became the default architecture for first movements of symphonies, sonatas, string quartets, and concertos for over a century. Beethoven stretched it into vast arguments — the first movement of the 'Eroica' Symphony (1804) is sonata form expanded to forty per cent longer than any earlier symphonic movement. Schubert, Brahms, Mahler, and even Bartók wrote sonata-form movements. Understanding the form is one of the keys to hearing how Western art music thinks.