contrary to all naturalness in song. True the human voice
trembles of itself, but only in a degree that remains beautiful;
it is in the nature of the voice. We imitate it not only on wind
instruments but also on the viols and even on the clavier. But as
soon as you overstep the limit it is no longer beautiful because
it is contrary to nature."

(Paris, June 12, 1778, to his father. [The statement that the
tremolo effect could be imitated on the clavier seems to require
an explanation. Mozart obviously had in view, not the pianoforte
which was just coming into use in his day, but the clavichord.
This instrument was sounded by striking the strings with bits of
brass placed in the farther end of the keys which were simple and
direct levers. The tangents, as they were called, had to be held
against the strings as long as it was desired that the tone
should sound, and by gently repeating the pressure on the key a
tremulousness was imparted to the tone which made the clavichord
a more expressive instrument than the harpsichord or the early
pianoforte. The effect was called Bebung in German, and
Balancement in French. H.E.K.])

66. "Before dinner Herr Vogler dashed through my sonata prima
vista. He played the first movement prestissimo, the andante
allegro and the rondo prestissimo with a vengeance. As a rule,
he played a different bass than the one I had written, and
occasionally he changed the harmony as well as the melody. That
was inevitable, for at such speed the eyes can not follow, nor
the hands grasp, the music. Such playing at sight and...are all
one to me. The hearers (I mean those worthy of the name) can say
nothing more than they have seen music and clavier playing. You