The "Air on the G String" is the second movement of Bach's Orchestral Suite No. 3 in D major (BWV 1068), composed around 1730, and was famously arranged for violin and piano by August Wilhelmj in the 19th century so that it could be played entirely on the violin's lowest string. Its serene, cantabile melody has made it one of the most beloved pieces in the Baroque repertoire.
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