Praetorius wrote a book about music theory called "Syntagma Musicum". Part One talks about religious music. It is very interesting for us today because it tells us a lot about the way that Martin Luther wanted to change music in the church services. In Part Two he described the musical instruments of his day. Part Three talks about musical forms: this included discussions about things like music notation (the way music was written), transposition (music), solmization and how to write for large choirs. He was going to write a Part Four in which he wanted to discuss the technique of musical composition, but he died before he could write it.