Mark Everist is Professor of Music at the University of Southampton. His previous publications include Mozart's Ghosts: Haunting the Halls of Musical Culture (2013), The Cambridge Companion to Medieval Music (Cambridge 2011), and French Motets in the Thirteenth Century (Cambridge 1994). Thomas Forrest Kelly is Morton B. Knafel Professor of Music at Harvard University, and has published numerous works including Early Music: A Very Short Introduction (2011), The Exultet in Southern Italy (1997) and the Kinkeldey Award-winning The Beneventan Chant (Cambridge 1989).