 The collection contains six arias with variations, all on original, secular themes.The practice of composing variations on original themes was a relatively new one (one previous instance was Frescobaldi's "Aria detta la Frescobalda" from the 1627 "Secondo libro di toccate"; of Pachelbel's contemporaries Bernardo Pasquini was one of the main exponents of this trend), and Pachelbel was among the first in Europe to explore the form.The overall plan of "Hexachordum Apollinis" is as follows