For a while now I've been fascinated by the style of chant known as "organum". For those of you not familiar with the form, it takes the melodic line of chant, and by adding one or two, or more voices contextualises the chant with basic underpinning harmony. In Medieval times this practice began to take hold and the cantus firmus would be sung by a tenor with (usually) a bass/bourdon singing a sort of continuo in parallel fourths and perfect fifths underneath the chant.
It wasn't ever really intended as "polyphony" as such, but was used in the tropes and chants of the masses of the greater feasts as a way of enhancing and highlighting the importance of the liturgy. Early treatise exist on the method and practice of Organum, the earliest I can find reference to being the Musica enchiriadis, believed to have been written around 895AD and covering topics such as njotation, modes, and monophonic plainchant. That the treatise was wrongly ascribed to Hucald, and then later Odo of Cluny suggests that the use of Organum had at least the tacit approval of the church authorities at the time.
Over the years the styles of Organum developed from simple idiaphonia to melismic writing from Notre Dame and Limoges in the 12th Century that would be almost indistinguishable from early polyphony.
As a style it is being used still in liturgical settings, with ensemble Exsurge Domine singing it regularly at the traditional masses in the Basilica Magistrale di S. Croce a Cagliari. I, for one, find it absorbing.
Kyrie IV Cunctipotens