If you could take a time machine to 1170 and attend mass at the Cathedral de Notre Dame, you might not be surprised to hear plainchant, but two French composers, Leonin and Perotin, were expanding on those chants and rearranging them for multiple voice parts in a new style called "organum."

Organum.  A subject that plants fear into the hearts of music students across the nation.  We are required to learn it, but the good news is, it doesn't require a PhD to understand it and to enjoy listening to it.

Here is the recipe. You begin with a plainchant, in which the soloist sings the verse and the chorus, or schola, picks up the refrain.

In the organum version, the solo sections are replaced by two voices.  One is singing long notes of the original chant, and the other sings an ornamental melody above it.  At certain points, the singers go from a free rhythm into a very regular rhythm.  This is called a discant.

This new style was composed to be sung in a liturgical setting, but, these beautiful complex pieces have inspired instrumentalists as well as singers, resulting in some very interesting renditions, such as those found on the Grammy-nominated CD entitled "Early Music: Lachrymae Antiquae," from the Kronos Quartet.

First, listen to a bit of the original "Notre Dame Organum," circa 1200, performed by the Deller Consort of London in 1961:

Next, listen to the same piece arranged for four a cappella voices, performed by Theatre of Voices, and finally, the string quartet version by the Kronos Quartet:

For more recordings of organum, check out the Hilliard Ensemble's release of music by Perotin, appropriately entitled "Perotin," the Orlando Consort's "Mystery of Notre Dame" and "The Age of Cathedrals" from the Theatre of Voices, directed by Paul Hillier.