Alleluia: Justus ut palma, Early experiment with polyphony.

This is an example of the earliest experiments with polyphony, two or more semi-independent musical melodies sung together. It is called "organum." The first organa used parallelism in the second musical line, that is, the second line was a fourth or fifth below the main chant line. In this example of free organum, the intervals above the main chant line vary in their distance, from unisons to fifths. The melody above the chant keeps the same rhythms, so the text is easy to understand.