Did Mozart really write whole pieces in his head?

Not quite. The legend says that Mozart heard whole works complete in his head and merely copied them out, but the letters and manuscripts tell a more interesting story. He did often work in his head, but he also made sketches, hired keyboards, and sometimes ran out of time.

The legend has a source. The early 19th-century publisher Friedrich Rochlitz printed a letter, supposedly by Mozart, in which a work "stands almost finished and complete in my mind", surveyed "like a fine picture or a beautiful statue, at a glance". That letter is now considered fraudulent, yet it was relayed by authorities such as the mathematician Henri Poincaré and it had a great influence on the popular picture.

What the authenticated documents show is a working composer. About 320 sketches and drafts survive, covering about 10 percent of his work, and many more were destroyed by his widow. Scholars have found a system: quick snippets, then drafts of the melody and often the bass, then a draft score, then the filling in of the inner voices to make a clean final score. That explains a letter of December 1780 about the opera Idomeneo, in which he is writing "at breakneck speed" because "everything's composed—but not written yet".

His real gifts were still extraordinary: an excellent musical memory, though probably not the miracle of legend, and a prodigious talent for improvising. Scholars think that to meet a deadline he left part of a piano concerto unwritten and improvised it in front of the audience.