Principal chamber music genre of the Baroque era. Despite its name, it requires four performers: two melody instruments and continuo (usually a keyboard instrument and a bass instrument). It arose early in the 17th century as an instrumental version of the Italian vocal-duet ensemble. The two upper instruments, usually violins, generally wove their melodic, quasi-vocal lines high above the accompanying parts. Two standard forms emerged after 1750: the sonata da chiesa, or church sonata, standardized as a four-movement form (in slow-fast-slow-fast order); and the suite-like sonata da camera, or chamber sonata. By 1770 the genre had been abandoned in favour of the solo sonata.
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A trio sonata is written for two solo melodic instruments and basso continuo, making three parts in all, hence the name trio sonata. However, because the basso continuo is usually made up of at least two instruments (typically a cello or bass viol and a keyboard instrument such as the harpsichord), trio sonatas are typically performed by at least four musicians. The trio sonatas by Arcangelo Corelli (opus I, 1681, opus III, 1689) set an inspiring example.
Johann Sebastian Bach's trio sonatas for organ (BWV 525-530) combine all three parts on one instrument. Typically the right hand, left hand and pedals will each take a different part thus creating the same texture as in a trio. For obvious reasons, these six trios have been transcribed for four musicians in recent times. A further innovation of Bach was the creation of what are strictly trio sonatas, involving a concertante (obligato) harpsichord part and one melodic instrument, thus for two players. Known examples are the six sonatas for harpsichord and solo violin (BWV 1014-1019), three sonatas for harpsichord and viola da gamba (BWV 1027-1029) and the three sonatas for harpsichord and flauto traverso (BWV 1030-1032)