Ornamentation (music)
The correct way of ornamenting music varied a lot from one country to another and from one century to another.  Ideas about how music should be performed kept changing.  It is important for musicians today, who want to play music from these older periods, to know as much as possible about historical musical styles.  Sometimes we have to make guesses about what a composer might have wanted.  Fortunately several composers and music theorists wrote books about how to play ornaments.  This enables us to understand the different performing styles.  Sometimes composers wrote a preface (introduction) in their music to explain to the performer how to play the ornaments they had written.