'''six sonatas for keyboard and violin, K. 26-31''' were composed in early 1766 in The Hague during the Mozart family's grand tour of Europe. They were dedicated to Princess Caroline of Nassau-Weilburg on the occasion of the eighteenth birthday of her brother, the Prince of Orange.Sadie, Stanley. Mozart: The Early Years, 1756-1781, p. 94-108, New York: W.W. Norton & Co. (2006) ISBN 0-393-06112-4. They were published as Mozart's "Opus 4".
These works show an improvement in compositional technique over the sets for Paris (KV 6-9) and London (KV 10-15), although like the previous sets, the keyboard part dominates and the violin may be considered optional.
Mozart composed all three early sets of accompanied sonatas while touring northwest Europe. These types of sonatas were not favored at home in Salzburg. Mozart would not revisit this genre until 1777-78 on a trip to Mannheim and Paris.