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. 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 role of the three instruments 
The piano trios of the Classical era, notably those of Haydn, are dominated by the piano part. The violin only plays the melody a certain amount of the time, and is often doubled by the piano when it does. 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 earlier trios are also rather dominated by the piano part.
With time, a new ideal of piano trio composition arose, in which each of the three instruments was supposed to contribute equally to the music. This is seen, for instance, in Beethoven's trios, and was likely in part the result of the increase in the power and sonority of the piano that took place during Beethoven's career, making it more feasible for the piano to play independently in an ensemble. 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.
Other combinations 
Some rather rare combinations of instruments have nonetheless inspired a few outstanding works.
- Haydn wrote three trios for flute, cello & piano (H. 15/15-17), a combination for which Carl Maria von Weber also wrote one work (op. 63).
- Beethoven wrote his Trio in G major, WoO 37 (1786) for flute, bassoon, and piano.
- Francis Poulenc's Trio op. 43 (1926) is for oboe, bassoon and piano.
- The Horn-violin-piano trio is exemplified by Brahms' Trio Op. 40 in E flat and György Ligeti's 1982 Trio for Violin, Horn and Piano.
- Trios with clarinet include masterpieces by Mozart, Beethoven, Brahms and Bartok; many more works are listed in the articles Clarinet-violin-piano trio, Clarinet-viola-piano trio and Clarinet-cello-piano trio.
- Ignaz Lachner wrote all of his six piano trios for violin, viola, and piano.
Example piano trios, extant and defunct 
Among the best known of such groups are or have been:
- Altenberg Trio (Austria)
- Maria Baptist Trio (Berlin, Germany)
- Istomin-Stern-Rose Trio, consisting of Eugene Istomin, Isaac Stern and Leonard Rose. (United States)
- One consisting of Alfred Cortot, Jacques Thibaud and Pablo Casals, earlier in the 20th century
- The Beaux Arts Trio (United States), whose commitment to using the same players in every concert pioneered a new generation of similarly committed groups; defunct
- The Sitkovetsky Trio (United Kingdom) consisting of Alexander Sitkovetsky, Wu Qian and Leonard Elschenbroich
- Trio di Trieste (Italy) consisting of Dario De Rosa, Renato Zanettovich and Libero Lana/Amedeo Baldovino; defunct
- The "Ax-Kim-Ma" trio, consisting of Emanuel Ax, Young Uck Kim, and Yo-Yo Ma (United States)
- Eroica Trio (United States)
- Ahn Trio (United States/Korea)
- Trio Fontenay (Germany)
- Suk Trio (Czech Republic)
- The Florestan Trio (United Kingdom)
- The Greenwich Trio (United Kingdom)
- The Oberlin Trio (United States)
- The Alma Trio (United States)
- Trio Wanderer (France)
- Haydn Trio Eisenstadt (Austria): Harald Kosik, Hannes Gradwohl, Bernd Gradwohl/Verena Stourzh.
- Bamberg Trio (Germany)
- Manhattan Piano Trio (United States)
- Spirale Piano Trio (Belgium)
- Xonor Trio (United States)
Famous works for piano trio 
- Anton Arensky's Piano Trio No. 1 in D minor
- Ludwig van Beethoven's trios, especially Piano Trio No. 7 in B-flat major "Archduke", Op. 97
- Johannes Brahms's Piano Trio No. 1 in B major, Op. 8 and No. 2 in C major, Op. 87
- Ernest Chausson's Piano Trio in G minor, Op. 3
- Frédéric Chopin's Piano Trio in G minor, Op. 8
- Antonín Dvořák's Piano Trio No. 4 in E minor ("Dumky"), Op. 90
- Gabriel Fauré's piano trio, Op. 120
- Joseph Haydn's 45 piano trios, particularly those composed from the mid-1780s onwards
- Felix Mendelssohn's two piano trios
- Wolfgang Amadeus Mozart's piano trios, particularly K502, K542 and K548
- Sergei Rachmaninoff's Elegiac Trios No. 1 in G minor and No. 2 in D minor
- Joachim Raff's 5 piano trios: in G minor, WoO 9 (1849), in C minor, Op. 102 (1861), in G major, Op. 112 (1863), in A minor, Op. 155 (1870), in D major, Op. 158 (1870)
- Maurice Ravel's Trio for Piano, Violin and Cello
- Camille Saint-Saëns's Piano Trio No. 2 in E minor
- Franz Schubert's Piano Trio No. 1 and No. 2
- Robert Schumann's Piano Trio No. 1 in D minor
- Dmitri Shostakovich's Piano Trio No. 2 in E minor
- Bedřich Smetana's Trio for Piano, Violin and Cello
- Pyotr Ilyich Tchaikovsky's Piano Trio in A minor, Op. 50
See also 
- Wheelock (1999 115)
- See Rosen 1997, 353
- Parakilas, James (1999) Piano roles : three hundred years of life with the piano. New Haven, CT: Yale University Press.
- Rosen, Charles (1997) The Classical Style: Haydn, Mozart, Beethoven. New York: Norton. Chapter VI.2 covers the trios in detail.
- Wheelock, Gretchen (1999) "The classical repertory revisited: instruments, players, and styles," in Parakilas (1999), pp. 109–131.