Franz Hoffmann, Mozart's Youth
“ He sat down to the piano, and Princess Adelaide, who was really a fine singer, began the cavatina. Wolfgang accompanied her, imperfectly of course, and sometimes incorrectly, for, as every musician knows, it is almost impossible to divine every modulation and digression in an unknown melody. The princess had hardly ended before Wolfgang requested her to repeat the cavatina, and this time accomplished what human ears had seldom, if ever, heard before. He not only played the melody with his right hand, but the bass accompaniment with his left, apparently with the greatest facility. ”
