Johann Sebastian Bach's Brandenburg Concerto No. 6 in B flat major, BWV 1051 is the final concerto in a set of works dedicated to Christian Ludwig, Margrave of Brandenburg. (It may actually have been the first composed, however.) They were intended as a job application, but the job did not appear.

Bach's sonic imagination was seemingly limitless, and for this final concerto he chose to limit the work's instrumentation to strings and continuo, meaning that the only non-bowed instrument heard is the harpsichord. Every other concerto in the set made extensive use of contrasting timbres, balancing the strings with the winds, often in unprecedented ways. This limitation of timbre is also extended to register; there are no violins, just two violas, two violas da gamba, a cello, and the violone, which is near the cello range and is from the gamba family. The overall effect of this decision is a spirit of repose and conclusion. There are no visceral contrasts in the music, though the final Allegro is faster than the other two movements; the concerto, whenever it was actually composed, makes a splendid way to end the overall set.

Bach's writing for these instruments was unconventional for the time. In the early eighteenth century the lower members of the violin family were considered orchestral instruments with supporting roles. They were given comparatively easy parts to play, while the gamba and its relatives were regarded as chamber instruments and necessarily received more difficult lines. Bach chose to reverse the level of difficulty, giving the viola and cello the tough solo parts, while the gamba players were free to cruise along in the supporting roles. In the second-movement Adagio, they are completely silent.

The form of the three-movement work is also filled with reversals. The opening movement sounds initially like a freely composed fugal arrangement, free of the stark contrasts normally associated with concerto form. Its ritornello, normally a focused bit of recurring melody, rambles along without drawing much attention to itself, while the music that is supposed to be spun out of the ritornello is concise and sharp. Compounding the irregularities further, the second movement (lovely and languid) ends in a different key from the one it starts in. The final movement assumes the character of a fugal gigue, but reveals itself to be a set of variations based on the initial ritornello, which is a much freer demonstration than the traditional spinning-out of the initial material.

Overall, these surprises result in what in many ways is the most various and striking among the Brandenburg Concertos. Its beauty is equal to its invention.

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