A string trio is a group of three string instruments or a piece written for such a group. The earliest string trio form consisted of two violins and cello, a grouping which had grown out of the baroque trio sonata.
Since the early classical period, the most common string trio configuration in classical chamber music has consisted of a violin, a viola and a cello. One of the first great masterworks for string trio is the Divertimento in E-flat, K.563, by Wolfgang Amadeus Mozart. In it Mozart sublimely melds together the concertante style typical of composers such as Luigi Boccherini, Carl Ditters von Dittersdorf, Felice de Giardini, and Alessandro Rolla with the more dynamically conversational style of composition perfected by Haydn in his string quartets. Ludwig van Beethoven wrote five string trios. His first, Op.3 in E-flat, is an ambitious six-movement work that was very likely inspired by Mozart's K.563. His second, the Serenade in D, Op.8, shows Beethoven experimenting with more extreme characters and unusual uses of form. In Beethoven's final three trios, Op.9 Nos.1-3, the young composer speaks with both intense passion and stunning authority. They are considered to be highlights of his early style and masterpieces in their own right.
Writing for a violin, viola and cello trio provides a wide palette of textures and colors for a skilled composer. The leaner instrumentation (as compared to the more common string quartet) also poses compositional challenges especially within a musical tradition typified by four-part harmony writing. In the 19th and 20th centuries countless composers after Mozart and Beethoven have successfully taken up this challenge, including Franz Schubert, Heinrich von Herzogenberg, Richard Strauss, Sergei Taneyev, Ernst von Dohnányi, Max Reger, Eugène Ysaÿe, Alexis Roland-Manuel, Miklós Rózsa, Arnold Schoenberg, Anton Webern, Paul Hindemith, Jean Sibelius, E J Moeran, Bohuslav Martinů, Darius Milhaud, Henry Cowell, Gideon Klein, Jean Francaix, Lennox Berkeley, William Schuman, Robert Simpson and Heitor Villa-Lobos. More recently, string trios have been written by Alfred Schnittke, Krzysztof Penderecki, Iannis Xenakis (Ikhoor), Krzysztof Meyer, Bertold Hummel, Wolfgang Rihm, Brian Ferneyhough, Karlheinz Essl, Murray Adaskin, Robert Carl, Talivaldis Kenins, Nick Norton, David Macbride, Thomas Schuttenhelm, James Wintle and Graham Waterhouse.
While string trio ensembles are certainly more rare than string quartets, there have been and continue to be ensembles dedicated to performing and recording the string trio repertoire. The Pasquier Trio and the Trio Italiano d'Archi were internationally renowned in the twentieth century and currently groups such as the Aspen String Trio, the Adaskin String Trio and the Leopold String Trio carry on this tradition.
Examples of more unusual string trio configurations include Wolfgang Amadeus Mozart's trio for two violins and double bass, and Antonín Dvořák's trio for two violins and viola. This sort of trio is known as a terzet or terzetto, and others who have written trios for this combination including Robert Fuchs (three, two in his opus 61 and one in his opus 107) and Sergei Taneyev (his op. 21.) The Masada String Trio, a group that performs the music of John Zorn, is configured for violin, 'cello and double bass.
