My highschool music textbook does not have "bimodal" in the index (spect I'll have to go to a statistics book for that). But it does afford:

[O]ne of the first forms of melodic combination [was] a system in which one melody was paralleled by another melody removed from it by intervals, which were most frequently, but not exclusively, fourths and fifths. This procedure was in marked contrast to classical and modern music, which considers thirds and sixths to be consonances. Melodic combination based on fourths and fifths was called _organum_....