The Brandenburg Concertos were originally written, as modern research has established, for the Köthen Court Orchestra, of which Bach was conductor. The instrumentation of the concertos corresponds exactly with the composition of the orchestra. The expert musicians necessary for the extremely difficult solo parts were employed here as "Cammer Musici": the violinist Josef Spiess, the oboist Friedrich Rose, the flautists Heinrich Freitag und Johann Gottlieb Würdig, the trumpeter Johann Ludwig Schreiber. Horn players, who were needed for the first of the concertos, were not permanently employed; there were, however, often travelling horn virtuosos (always in pairs) at Köthen, where they wished to be heard. The gamba was played by Christian Ferdinand Abel and the Prince himself the ’cello by Christian Bemard Linigke. In addition, a further violinist and a bassoon player were also employed as "Cammer Musici", and there were also four other ordinary "Musici". We can thus form an accurate mental picture of the orchestral forces Bach had "in his ear" when he wrote the Brandenburg Concertos, and how they were originally played.
Much has been written about the formal relationship of these concertos to the type of the Italian Concerto grosso. As in nearly all other cases too, Bach has, in these concertos, made great changes to the traditional form in order to adapt it to suit his ideas. The contrast between tutti and solo is, of course, here too the decisive element in the form of the works; what is new is the freedom and variety with which these media are applied, and in addition to conventional solo passages accompanied by the strings or the continuo, there are others in which all instruments are involved with equal importance. Consequently these stand out from the more massive tutti blocks only by virtue of their part writing and tone quality. The instrumentation, too, goes further than anything know up till then: in the First Concerto we find two concerto horn parts for the first time in musical history, the Second is the only work that has come down to us in which a high F trumpet is used. The Fifth is the first concerto known to us for a keyboard instrument. There are no previous models for the selection of instruments found in the Third, Fourth and Fifth Concertos; their original instrumentation, which goes to the very limits of the possible, indicates the importance of the various components of tone character in the performance of these works.
The oldest stilistic character is to be found in the Sixth Concerto; alone its instrumentation with only low string instruments recalls the English consorts of viols in the 17th century. The actual solo instruments are the violas. We know that Bach himself enjoyed playing the viola, and this is surely the reason why he has written such a gratifying double Concerto for this typical accompaniment instrument, for which hardly any solo compositions existed up till then. By giving the middle parts to gambas he has avoided the danger of thick masses of tone in these lower registers, which the ear could not unravel, V/ith their bright, transparent tone quality, the gambas stand out clearly against both the two solo violas and the virtuoso solo 'cello part. Limited to ostinato continuo chords in the tutti passages, they participate in the play of motifs in the solo sections.
In the Fifth Brandenburg Concerto, the earliest known work with a concerto part for the harpsichord, all the technical and tonal possibilities of this instrument are employed in such a masterly fashion that this work becomes at the sarne time the beginning and the climax of its category, The two other solo instruments, flute and violin, leave the harpsichord the dominant role in the first movement, this being underlined by a rnagnificently composed solo cadenca, Although the second movement is reserved for the three solo instruments alone, the contrast between "solo" and "tutti" is strictly observed: the tutti passages are marked "forte", the harpsichord here only playing the continuo accompaniment, whereas the solo passages are "piano" with the right hand of the harpsichord playing the most important of the three upper parts.
ln the Second Concerto the trumpet is usually predominant in everybody's mind and this although the four solo parts are musically of equal importance in this concerto more than in any other. The primacy of place given to the trumpet part results from its well-known difficulties and extremely high range and, obove all, from the fact that the modem trumpet dominates to such an extent that this work must be felt to be a trumpet concerto. At the same time, the second solo instrument is a recorder, the softest of all wind instruments! lt is here that the correct balance of the original instruments is revealed most clearly. The natural trumpet sounds so soft and gently that it never pushes its way into the foreground. On hearing the work played thus, we realize that the solo quartet has been chosen on account of the four entirely different and characteristic tone colours. Their variety is demonstrated in the solo passages, their wonderfully colourful blend when they play together.