Works titled "Piano Trio" tend to be in the same overall shape as a
sonata. Initially this was in the three movement form, though some of
Joseph Haydn's have two movements. In five late works,
Wolfgang Amadeus Mozart is generally credited with transforming the accompanied keyboard sonata, in which the essentially optional cello doubles the bass of the keyboard left hand, into the balanced trio which has since been a central form of chamber music. With the early 19th century, particularly
Ludwig van Beethoven, this genre was felt to be more appropriate to cast in the four movement form. Piano trios that are set in the
Sonata tradition share the general concerns of such works for their era, and often are reflective directly of symphonic practice with individual movements laid out according to the composer's understanding of the
sonata form. In the
Classical era, home music-making made the piano trio a very popular genre for arrangements of other works. For example, Beethoven transcribed his first two symphonies for piano trio. Thus a large number of works exist for the arrangement of piano, violin and violoncello which are not generally titled or numbered as piano trios, but which are nonetheless part of the overall genre. These include single movements as well as sets of variations such as Beethoven's
Kakadu Variations Op. 121a and
Variations in E flat major Op. 44. After the Classical era, works for piano and two instruments continue to be written which are not presented as in the sonata tradition, or are arrangements of other works. Many of these individual works are popular on concert programs, for example
Joseph Suk's
Elegy. ==The role of the three instruments==