eli5: What is “cantus firmus”?


I am doing research for a school academic team, but I cannot find a good explanation or example of it to help me understand. I know that it’s a polyphonic? However, I don’t really know what that is either, so that doesn’t really help. Any help would be appreciated :).

In: 4

Two important things for me to define are monophonic and polyphonic textures of music. A monophonic texture is where there is just a single melody and nothing else. [Here is an example](https://www.youtube.com/watch?v=kK5AohCMX0U)

A polyphonic texture is where there are multiple independent melodies happening at the same time. Think of something like a Bach Fugue where there are different “voices” happening at the same time.

If you are familiar with Gregorian chant or plainchant, it is a monophonic chant that is sung in unison. As chant developed further, most notably at the Notre Dame Cathedral, it slowly began to become more complicated.

The cantus firmus is the “fixed song” or original chant that the song is based on. The original chant undergoes augmentation (the duration of each syllable is increased dramatically), and this line that is sung is the cantus firmus. These syllables become so long that they essentially act as drones that periodically change pitch.

On top of the cantus firmus are anywhere from one to three voices that, when combined, create a polyphonic texture.

[Here is a video that has some good information about this with a nice comparative example between the organum of two different composers in Notre Dame.](https://www.youtube.com/watch?v=WSsutCu8PIo)

I am by no means an expert, this is just what I learned in a music history class a few years ago.