Part 1 Introduction: status and description of "De pratica"; Guglielmo's life; dancing in 15th-century Italian society; the music in "De pratica": transcribing the "Balli" tunes. Part 2 "De pratica seu arte tripudii" - "On the Practice or Art of Dancing". Appendices: additions from the Giovanni Ambrosio copy of "De pratica" - on dancing, choreographies and music; autobiography.