Basso continuo parts, almost universal in the Baroque era (1600–1750), provided the harmonic structure of the music. The phrase is often shortened to continuo, and the instrumentalists playing the continuo part, if more than one, are called the continuo group. The titles of many Baroque works make mention of the continuo section, such as J. S. Bach's Concerto for 2 violins, strings and continuo in D minor.
The makeup of the continuo group is often left to the discretion of the performers, 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, or harp. In addition, any number of instruments which play in the bass register may be included, such as cello, double bass, bass viol, or bassoon. The most common combination, at least in modern performances, is harpsichord and cello for instrumental works and secular vocal works, such as operas, and organ for sacred music. Very rarely, however, in the Baroque period, the composer requested specifically for a certain instrument (or instruments) to play the continuo. In addition, the mere composition of certain works seems to require certain kind of instruments (for instance, Vivaldi's Stabat Mater seems to require an organ, and not a harpsichord).
The keyboard (or other chording instrument) player realizes a continuo part by playing, in addition to the indicated bass notes, upper notes to complete chords, either determined ahead of time or improvised in performance. The player can also "imitate" the soprano (which is the name for the solo instrument or singer) and elaborate on themes in the soprano musical line. 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 as a guide. Modern editions of music usually supply a realized keyboard part, fully written out for the player, eliminating the need for improvisation. With the rise in historically informed performance, however, the number of performers who improvise their parts, as Baroque players would have done, has increased.
Basso continuo, though an essential structural and identifying element of the Baroque period, continued to be used in many works, especially 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, Beethoven, and Franz Schubert, for example, have a basso continuo part for an organist to play.
Read more about this topic: Figured Bass