<P> After the closing cadence, the musical argument proper is said to be completed harmonically . If the movement continues, it is said to have a coda . </P> <P> The Coda is optional . After the final cadence of the recapitulation, the movement may continue with a coda which will contain material from the movement proper . Codas, when present, vary considerably in length, but like introductions are not part of the "argument" of the work . The coda will end, however, with a perfect authentic cadence in the original key . Codas may be quite brief tailpieces, or they may be very long and elaborate . An example of the more extended type is the coda to the first movement of Beethoven's Eroica Symphony, and an exceptionally long coda appears at the end of the finale of Beethoven's Symphony No. 8 . </P> <P> Explanations for why an extended coda is present vary . One reason may be to omit the repeat of the development and recapitulation sections found in earlier sonata forms of the 18th century . Indeed, Beethoven's extended codas often serve the purpose of further development of thematic material . Another role that these codas sometimes serve is to return to the minor mode in minor - key movements where the recapitulation proper concludes in the parallel major, as in the first movements of Beethoven's Symphony No. 5 or Schumann's Piano Concerto, or rarely, to restore the home key after an off - tonic recapitulation, such as in the first movements of Brahms's Clarinet Quintet and Dvořák's Symphony No. 9 . </P> <P> It is not necessarily the case that the move to the dominant key in the exposition is marked by a new theme . Haydn in particular was fond of using the opening theme, often in a truncated or otherwise altered form, to announce the move to the dominant, as in the first movement of his Sonata Hob . XVI No. 49 in E ♭ major . Mozart also occasionally wrote such expositions: for instance in the Piano Sonata K. 570 or the String Quintet K. 593 . Such expositions are often called monothematic, meaning that one theme serves to establish the opposition between tonic and dominant keys . This term is misleading, since most "monothematic" works have multiple themes: most works so labeled have additional themes in the second subject group . Rarely, as in the fourth movement of Haydn's String Quartet in B ♭ major, Op. 50, No. 1, did composers perform the tour de force of writing a complete sonata exposition with just one theme . A more recent example is Edmund Rubbra's Symphony No. 2 . </P>

The three major sections of sonata form is exposition development coda