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 Third Concerto, consisting only of two quick movements, is a splendid demonstration of pure string tone. Three violins, three violas and three cellos are contrasted with each other as groups. Solo and tutti passages are skilfully interwoven with one anotherin the first movement, each player coming forward as a soloist from the group several times. Two chords lead into the second movement; in accordance with the old custom they have to be embellished by a little cadenza. In this purely string concerto it should naturally be played by a string instrument rather than by the harpsichord, which here has only an accompanying function. In the second Allegro it is only the first violin and the first viola that are honoured as soloists, but it forms a finale which is a unique demonstration of collective virtuosity.
The most remarkable work as regards form is the First Concerto. A normal concerto in three movements is followed by an almost independent sequence of dances grouped around a minuet. The instrumentation is markedly treated as three choirs of instruments: strings, oboes with bassoon and a pair of horns. The latter have not been built homogeneously into the ensemble; the horn players, after all, were not members of the court orchestra, and their performances inevitably awakened hunting associations. Bach emphasizes this musical character by introducing the horns in a hunting call in triplet rhythm, which they add as if "from outside" to the first tutti ofthe ensemble proper. The violin solos in the second and third movements must be played on a piccolo violin, tuned a third higher than the normal instrument. Bach demands this instrument purely for the sake of its tone colour; there are no
technical reasons why it should have been chosen. The very delicate and yet incisive tone of this violin contrasts wonderfully with that of the oboes as also with that of the other string instruments. In the velvet-smooth tone of the string Polonaise, it is silent.
The Fourth Concerto is, in the first place, a violin concerto. On account of the echo effects in the second movement, Bach calls the two solo recorders "Flauti d’Echo". ln this movement the quaver notes are played rhythmically unevenly, in accordance with former practice in performance (Quantz wrote in 1752: "...the notes must... be played slightly unevenly... so that the first, third and fifth are held somewhat longer than the passing notes... though not as long as if dots stood beside them...").