How did contrary motion make the organal voice more independent from the chant?

In the development of medieval polyphony, the use of contrary motion was a significant step in moving away from the rigid constraints of early organum.

In the earliest forms of organum, the organal voice (vox organalis) was strictly subservient to the chant, or principal voice (vox principalis), typically moving in parallel motion at fixed intervals of a fourth or fifth. This practice resulted in a texture where the two voices were essentially locked together, lacking individual melodic character.123

As described in the 11th-century Micrologus by Guido of Arezzo, the introduction of contrary motion—along with oblique motion—allowed the organal voice to diverge from the chant's path. By moving in the opposite direction of the chant, the organal voice began to shed its role as a mere shadow or doubling of the principal melody. This evolution provided the organal voice with greater melodic individuality, marking a transition toward true polyphony where the parts could function as distinct, independent lines rather than simple ornaments of the chant.12