A musical work is a composition of music, typically written by a composer for performance by an ensemble or soloist. An opus is a musical work, usually numbered to indicate the order of the composer's works. A CANTATA is a vocal composition with an instrumental accompaniment, typically in several movements. An ORATORIO is a large-scale musical composition for orchestra, choir, and soloists, typically on a religious theme. A rondo is a musical form consisting of a recurring main theme alternating with different episodes. A DUET is a piece of music written for two performers. An OPERA is a dramatic work in one or more acts, set to music for singers and instrumentalists. A SYMPHONY is an extended musical composition in Western classical music, usually in four movements. A TRIO is a composition for three performers. An etude is a short musical composition, typically of considerable difficulty, designed as an exercise to improve the technique or execution of a particular aspect of playing an instrument. LARGO is a tempo marking indicating that music should be played slowly. A motet is a polyphonic choral composition, typically on a religious text. A NONET is a composition for nine performers. An OCTET is a composition for eight performers. A sonata is a musical composition, usually for a solo instrument or a small instrumental ensemble. A CONCERTO is a large-scale musical composition for orchestra and soloist or soloists. A RHAPSODY is a one-movement work that is episodic yet integrated, free-flowing in structure, featuring a range of highly contrasted moods, colour, and tonality. A SERENADE is a musical composition, often in several movements, performed outdoors at night.