A piano trio is a group of piano and two other instruments, usually a violin and a cello, or a piece of music written for such a group. It is one of the most common forms found in classical chamber music. The term can also refer to a group of musicians who regularly play this repertoire together; for a number of well-known piano trios, see below.
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 Haydn's have two movements. Mozart, in five late works, 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 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 period, 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 Variations on 'Ich bin der Schneider Kakadu' 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 Suk's Elegie.
For individual articles treating works for piano trio, see Category:Compositions for piano trio.
The piano trios of the Classical era, notably those of Haydn, are dominated by the piano part. The violin plays the melody only a certain amount of the time, and when it does, is often doubled by the piano. The cello part is very much subordinated, usually just doubling the bass line in the piano. It is thought that this practice was quite intentional on Haydn's part and was related to the sonority of the instruments of Haydn's day: the piano was fairly weak and "tinkling" in tone, and benefited from the tonal strengthening of other instruments. Mozart's five late (K 496 and later) trios are generally felt to mark the assured arrival of the form, attentive to balanced voices and three-part dialogue.
Beethoven's trios continued the compositional objectives inaugurated by Mozart. The new idea of equality was never implemented completely; the extent to which it is realized varies from one composition to the next, as well as among movements within a single composition. Certainly by the mid nineteenth century, all three instruments had been modified to have a very powerful sound, and each can hold its own in a modern ensemble.
The earlier trios are now frequently performed and recorded using authentic instruments, of the kind for which they were originally written. Such performances restore the sonic balance the composer would have expected, and have proven popular.
Some rather rare combinations of instruments have nonetheless inspired a few outstanding works.
Among the best known of such groups are or have been: