Trio sonata
Bach also wrote six trio sonatas for organ.  These are played by one player.  The organist’s two hands each play on different manuals (keyboards) so that they sound like two different instruments.  The organist’s feet play the third part on the pedals.  Each part is strictly a single line (no chords, never more than three notes at a time) so that these works can also be played by other instruments (e.g. two violins and cello with harpsichord).