In May 1747, Bach accepted a longstanding invitation to travel the roughly 100 miles from Leipzig to meet Frederick the Great in Potsdam, a long walk from Berlin. Bach was by then a legend among musical cognoscenti, a class that included the Prussian king, a fine flutist and capable composer who surrounded himself with eminent musicians. In 1740 he hired Bach’s second son, Carl Phillip Emanuel, who was his own age, to be the court’s “first keyboard player,” and almost immediately had C.P.E. communicate an invitation to the elder Bach, who took almost six years to accept. International politics may have made Bach cautious, particularly after Prussian forces occupied Leipzig during the Silesian Wars in 1745; a visit to Leipzig’s conqueror would not sit well with Bach’s employer, the Leipzig town council.
But the 62-year-old master was enthusiastically received when he arrived as Frederick’s regular royal evening chamber music sessions began. The king announced, “the elder Bach has arrived” and, with eagerness unusual in that formal age, had him immediately ushered into the music room without giving him time to change from his traveling clothes into attire suitable for royalty. A prominent Berlin newspaper reported four days later that the king then went “to the so-called ‘forte and piano,’ condescending to play, in person and without any preparation, a theme to be executed by Capellmeister Bach in a fugue.” In an age when “virtuoso” usually meant “someone who creates great music on his instrument,” the best musicians showed their mettle by improvising. “This was done so happily by the aforementioned Capellmeister that not only His Majesty was pleased show his satisfaction at it, but also all those present were seized with astonishment.” (This is one of the few times Bach is known to have played the pianoforte; Frederick, an early convert to the new hammer-action instruments, owned seven of them.) “Herr Bach has found the subject propounded to him so exceedingly beautiful that he intends to set it down on paper in a regular fugue and have it engraved in copper. On Monday, the famous man was heard on the organ in the Church of the Holy Ghost at Potsdam and earned general acclaim from the auditors attending in great number. In the evening, His Majesty charged him again with the execution of a fugue, in six parts, which he accomplished just as skillfully as on the previous occasion, to the pleasure of His Majesty and to the general admiration.”
After waiting six years for Bach to show up, Frederick would have certainly have put some thought into giving him a subject for improvisation that would challenge him to display depth and ingenuity, so we should be skeptical at being told the king came up with it on the spur of the moment. But Bach did indeed pay for an engraving of The Musical Offering, consisting of a dozen canons or fugues, as well as tonight’s trio sonata, on the “Royal Theme,” and sent it to the king. History does not record Frederick’s response.
While the rest of The Musical Offering explicitly develops the royal theme, the trio sonata mostly hints at it. The theme is not recognizable until it appears in the bass midway through the second movement. A variation of the theme becomes a jig in the last movement.
-- Notes by Howard Posner