Among the least known but most charming of the mature chamber music of Wolfgang Amadeus Mozart (1756-1791) are the two Duos for violin and viola, K. 423 and 424. Both were probably composed in Salzburg sometime between July and October 1783. Supposedly both were intended to be part of a six-piece collection of duets composed for Michael Haydn and commissioned by the Archbishop of Salzburg. The second of the two duets is slightly more expansive than the first in that the opening Allegro is preceded by an introductory Adagio and the closing movement is a set of variations on an original Andante grazioso theme. But the melodic charm, the contrapuntal inventiveness, and the sheer cleverness of Mozart's writing, which always seems absolutely perfect even though there are only two instruments, is consistently amazing and gives the Duo in B flat major a substance and integrity usually associated with quartets.

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