generations, the set of additional rules had become highly nuanced: Consonances were now categorized as perfect or imperfect, and the octave could be subdivided harmonically or arithmetically. The possible movement of simultaneous voices in relation to one another was carefully categorized into three types of motion—direct motion (two lines moving parallel), contrary motion (two lines moving in opposite, mirrored fashion), and oblique motion (one line moving while another remains stationary)—and the use of these motions was likewise determined by a set of rules relating perfect and imperfect consonances (Fux 1971, vii). One of the most interesting rules to have developed throughout this process, insofar as it provides a direct connection back to contemporary architectural discourse, is the use of the cantus firmus as the basis for contrapuntal composition. A cantus firmus (fixed song) is an existing melody taken from elsewhere and used as the basis for the composition with each additional voice composed fundamentally in relation to it and in relation to one another. It becomes the site of interpretation, the terra firma of the composition, as it were, making the entire compositional procedure explicitly contextual and interpretive (see Figure 12.4).