Perotin was a French composer who lived around late 12th century and early 13th century. He was one of the famous member of the Notre Dame School in polyphony and Ars Antiqua style.
This organ organum triplum from Perotin might be transcribed from the original vocal score. Most of this organum is a typical florid organum, which has two active top parts keep moving all the time, while the bottom pedal part is written in long notes which is called “tenor”. The florid organum is an advanced style in the early Middle Ages. From this organum triplum we can see, it is 12/8, which has a feeling of triple meter that was used almost exclusively at that period. The two upper voices move in all most the same rhythm in octaves, unison and open fifth. However, the consonance is not that perfect through the whole piece, major second and minor second happen sometimes indicate that dissonances was acceptable at that time. The tenor voice is the lowest, playing long notes below the motet. This pedal point is a symbol for the florid organum. Nevertheless, the tenor changes on measure 17-20. More active notes appear instead of long notes, what’s more, one note on tenor part matches three or four notes on the upper voices. This is the early counter point; also, it is the significant character for discant organum. On measure 25, all these three parts move in an exact same rhythm in different two octaves. This cadence section is called parallel organum.
Overall, based on the tenor, we have already found three different organums in this triplum: florid organum, discant organum and parallel organum.