Part IV of Le Istitutioni Harmoniche of Gioseffo Zarlino, on the modal theory of the 16th century, describing the attributes and use of the expanded set of 12 musical modes first described by Heinrich Glarean in 1547. Zarlino was the formost theorist of the 16th century, and the music director at St. Mark's in Venice for most of the 2nd half of the 16th century.