Bach's surviving works provide examples of concertos and suites, the two most important orchestral genres in the late Baroque. The composer prepared his final versions of the six Brandenburg concertos for presentation to the Margrave of Brandenburg in 1721, although each concerto had perhaps had earlier performances (at least two -- Nos. 1 and 5 -- in different versions) in Weimar or Cöthen. Each of the six features a different combination of solo and tutti instruments; Bach's specific groupings are highly unusual for the late Baroque. Numbers 2, 4, and 5 may be classified as concerti grossi, since each has a separate concertino and ripieno group. However, one solo instrument in each of these concertos stands out above the others in the concertino (the trumpet, violin, and harpsichord in Nos. 2, 4, and 5, respectively), thereby creating in effect three solo concertos. Numbers 1, 3, and 6 are more in the nature of ripieno or chamber concertos, since Bach does not designate a single soloist or solo group to play against a larger accompanying ensemble. These latter three works feature concertato interactions among ever-changing groupings of instruments (two horns, three oboes, bassoon, and strings in No. 1; three each of violins, violas, and cellos in No. 3; and two violas and two gambas in No. 6). All six concertos use as a point of departure the Vivaldi three-movement plan, with ritornello designs in fast movements, although No. 1 concludes with several additional dances and No. 3 lacks a slow middle movement. Bach's treatment of the ritornello movements is quite varied, often incorporating extensive contrapuntal imitations.
The four orchestral suites are now throught to have orginated in Cöthen (Nos. 1 and 4) and Leipzig (Nos. 2 and 3). Each has a different instrumentation and a different combination of dances following an opening French overture. Number 2 features solo flute throughout; Nos. 3 and 4 have a more festive character, with three trumpets and timpani.
