The first movement of the fifth one, in D major, contains a lengthy solo harpsichord cadenza that may have been performed by the composer himself at the works' premiere.  
An early version of the first movement of one of these works can be found in the composer's cantata Falsche Welt, dir trau ich nicht without the violino piccolo. One of these pieces contains a movement that only consists of a two- chord Phrygian half cadence, around which a harpsichord or violin improvises a long cadenza.  The sixth one has no violins, but has two (*) violas da gamba. The first movement of the fifth one, in D major, contains a lengthy solo harpsichord cadenza that may have been performed by the composer himself at the works' premiere.  FTP, name this collection of six musical works by J. S. Bach, named for a certain margave.
An early version of the first movement of one of these works can be found in the composer's cantata Falsche Welt, dir trau ich nicht without the violino piccolo.
{Brandenburg concertos}