a musical composition for three instruments or voices, or a group of three performers.

The term trio came to be identified with the middle section of a dance movement in ternary form (the b section of an aba form such as a minuet or a scherzo). The designation arose because many such trio sections were orchestrated for three instruments, such as in the “Minuet” of Johann Sebastian Bach (Bach, Johann Sebastian)'s Brandenburg Concerto No. 1 (1721; two oboes and bassoon) or Ludwig van Beethoven (Beethoven, Ludwig van)'s Symphony No. 8 (1812; two horns and cello section).

The typical trio sonata of the Baroque era comprised several movements for three instruments plus a basso continuo; the continuo instrument doubled the bass part and added harmonic support. A well-known trio sonata for flute, violin, and cello (with harpsichord) is part of Bach's Musical Offering (1747). Bach's Six Sonatas for organ (c. 1730) are for three contrapuntally balanced parts (two manual keyboards and pedal keyboard) without continuo.

In the Classical period the trio came into its own as a genre of chamber music. The string trio, normally for violin, viola, and cello, includes notable examples by Wolfgang Amadeus Mozart (Mozart, Wolfgang Amadeus) and Beethoven. Joseph Haydn (Haydn, Joseph)'s 20 string trios are for two violins and cello. Two notable 20th-century string trios are by Arnold Schoenberg (Schoenberg, Arnold) and Anton Webern (Webern, Anton). As the piano became more widely available in the 18th century, the piano trio (piano, violin, and cello), which makes possible a fuller and more varied texture, attracted the attention of composers. Haydn wrote nearly 40 of them; Beethoven (Beethoven, Ludwig van)'s piano trios, from the three of Opus 1 (1794–95) through the two of Opus 70 (1808) and the “Archduke” Trio in B-flat Major, Opus 97 (first performed 1810 or 1811), are considered some of his finest works of chamber music. Romantic composers of piano trios include Franz Schubert (Schubert, Franz), Johannes Brahms (Brahms, Johannes), and Antonín Dvořák (Dvořák, Antonín). In 1914 Maurice Ravel (Ravel, Maurice) wrote one of the best-known such works of the 20th century.

Trios for other combinations include Mozart's for clarinet, viola, and piano, K 498 (1786; known as the Kegelstatt Trio); Beethoven's Opus 11 (1798) for clarinet, cello, and piano; Brahms's Opus 114 (1891) for the same combination; and his Trio, Opus 40 (1865), for horn, violin, and piano. Many compositions for various trio combinations are not explicitly so labeled, such as Claude Debussy (Debussy, Claude)'s Sonata (1915) for flute, viola, and harp; Aaron Copland (Copland, Aaron)'s Vitebsk (1928) for piano trio; and Béla Bartók (Bartók, Béla)'s Contrasts (1938) for violin, clarinet, and piano.

In jazz, trios for any combination are common, but the fundamental jazz piano trio consists of piano, double bass, and drums. This grouping forms the core of most other small combos (i.e., quartets, quintets, etc.).