Basso continuo facet of Baroque music
Basso continuo parts, almost universal in the Baroque era (1600–1750), provided the harmonic structure of the music by supplying a bassline and a chord progression. The phrase is often shortened to continuo, and the instrumentalists playing the continuo part are called the continuo group.Forces
The composition of the continuo group is often left to the discretion of the performers (or, for a large performance, the conductor), and practice varied enormously within the Baroque period. At least one instrument capable of playing chords must be included, such as a harpsichord, organ, lute, theorbo, guitar, regal, or harp. In addition, any number of instruments that play in the bass register may be included, such as cello, double bass, bass viol, or bassoon. In modern performances of chamber works, the most common combination is harpsichord and cello for instrumental works and secular vocal works, such as operas, and organ  and cello for sacred music. A double bass may be added, particularly when accompanying a lower-pitched solo voice (e.g., a bass singer).
In larger orchestral works, typically performers match the instrument families used in the full ensemble: including bassoon when the work includes oboes or other woodwinds, but restricting it to cello and/or double bass if only strings are involved, although occasionally individual movements of suites deviate from this at the musical director's discretion (e.g. bassoon without oboes). Harps, lutes, and other handheld instruments are more typical of early 17th-century music. Sometimes instruments are specified by the composer: in L'Orfeo (1607) Monteverdi calls for an exceptionally varied instrumentation, with multiple harpsichords and lutes with a bass violin in the pastoral scenes followed by lamenting to the accompaniment of organo di legno and chitarrone, while Charon stands watch to the sound of a regal. Contrabassoon is rare as a continuo instrument, but is often used in J. S. Bach's Johannespassion which calls for "bassono grosso".The keyboard (or other chord-playing instrument) player realizes (that is, adds in an improvised fashion) a continuo part by playing, in addition to the notated bass line, notes above it to complete chords, either determined ahead of time or improvised in performance. The figured bass notation, described below, is a guide, but performers are also expected to use their musical judgment and the other instruments or voices (notably the lead melody and any accidentals that might be present in it) as a guide. Experienced players sometimes incorporate motives found in the other instrumental parts into their improvised chordal accompaniment. Modern editions of such music usually supply a realized keyboard part, fully written out in staff notation for a player, in place of improvisation. With the rise in historically informed performance, however, the number of performers who are able to improvise their parts from the figures, as Baroque players would have done, has increased.History
Basso continuo, though an essential structural and identifying element of the Baroque period, continued to be used in many works, mostly (but not limited to) sacred choral works, of the classical period (up to around 1800). An example is C. P. E. Bach's Concerto in D minor for flute, strings and basso continuo. Examples of its use in the 19th century are rarer, but they do exist: masses by Anton Bruckner, Ludwig van Beethoven, and Franz Schubert, for example, have a basso continuo part that was for an organist.
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