Das Musikalische Opfer

The Musical Offering (German title: Musikalisches Opfer or Das Musikalische Opfer), BWV 1079, is a collection of keyboard canons and fugues and other pieces of music by Johann Sebastian Bach, all based on a single musical theme given to him by Frederick the Great (Frederick II of Prussia), to whom they are dedicated. The Ricercar a 6, a six-voice fugue which is the highpoint of the entire work, was put forward by the musicologist Charles Rosen as the most significant piano composition in history (partly because it is one of the first). This ricercar is also occasionally called the Prussian Fugue, a name used by Bach himself. The composition features in the opening section of Douglas Hofstadter's Pulitzer Prize winning book Gödel, Escher, Bach (1979).

The collection has its roots in a meeting between Bach and Frederick II on May 7, 1747. The meeting, taking place at the King's residence in Potsdam, came about because Bach's son Carl Philipp Emanuel was employed there as court musician. Frederick wanted to show the elder Bach a novelty, the fortepiano, which had been invented some years earlier. The King owned several of the experimental instruments being developed by Gottfried Silbermann. During his anticipated visit to Frederick's palace in Potsdam, Bach, who was well known for his skill at improvising, received from Frederick a long and complex musical theme on which to improvise a three-voice fugue. He did so, but Frederick then challenged him to improvise a six-voice fugue on the same theme. The public present thought this was just a malicious caprice by the King, intent upon humiliating philosophers and artists. Bach answered that he would need to work the score and send it to the King afterwards. He then returned to Leipzig to write out the Thema Regium ("theme of the king"):

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