The manuscript of Bach’s Six Sonatas (BVW 1014-1019; 1720-1725) states “Six sonatas for cembalo certato and solo violin” – implying that the harpsichord does not merely play a basso continuo part, providing back-up chords, but actually “concerts” itself too, like in Arcangelo Corelli’s sonatas for three instruments. Another Italian reference is to the church style which, at least externally, is evident in Bach’s slow-fast-slow-fast sequence.
Händel’s irrepressible Sonata in D major bears the same mark, which makes it an ideal piece to open a program – like this one.
Briefly, therefore, Bach uses: 1) an introductory Adagio; for instance there is the splendid recitative that opens BWV 1014 or a slow dance like the sweet Siciliana that sets off BWV 1017; 2. an Allegro in fugue form; 3. an Adagio or an Andante, gently melodic and much more cantabile than the first movement; 4. another Allegro, an energetic fugue. 
Bach underpegs the Italian influence with a strong contrapuntal structure even in the cantabile and dance movements. This is clear in the Sonata in C minor BWV 1017. To start with there’s the Siciliana we just mentioned – a sort of aria for violin overlaid on a discreet bass line. In the Allegro the harpsichord sings out with an ample, discursive Fugue. Then comes the Adagio as third – another generous Largo. In the Finale, Allegro, the harpsichord starts the Fugue once more.
The Sonata BWV 1018 starts with a slow movement, Largo, the violin’s melody singing out over a pensive cantabile counterpoint. The Allegro is a dynamic, brilliant fugue. The violin plays a procession-like chant, with a graceful flowing accompaniment in the Adagio, immediately contrasted by the two instruments’ enthusiastic counterpoint in the Finale Vivace.
The Sonatas for violin and harpsichord show vividly that Bach’s compositions for string instruments reached a peak around 1720, illustrated in particular by his Six solos for violin without bass accompaniment (BWV 1001-1006; Köthen, 1720).
The Chaconne is the last movement of the Seconda Partita BWV 1004 for violin solo (a Partita is a suite of dances). It is so broad and stellar that it seems to just “devour” the previous pieces – the Allemande, Courante, Sarabande, Gigue. Variations on an eight-bar basso ostinato produce an unbelievable mix of violin and musical polyphony. The tension is continuous between fresh invention and the restrictions of the eight bars, and the violin expands beyond its usual “melodic” role to become “symphonic”.

Alberto Cantù