Music theory
In the 18th century some composers wrote books on music theory. Carl Philipp Emanuel Bach (son of the famous Johann Sebastian Bach) wrote a book called: “An Essay on the true art of playing the Keyboard”. Leopold Mozart (the father of the famous Wolfgang Amadeus Mozart) wrote a book called The Art of Playing the Violin. Both these books were extremely well known in their day. In spite of their titles the first halves of these two books are both about a branch of music theory called "performance practice": They tell us a lot about how music was played in those days, how some rhythms were played quite freely and how ornaments in the music were played.