Quintorganum
Quintorganum is a very early form of polyphony in Gregorian music from around the 9th century AD. The main melody was sung by the so-called principal voice. The same melody was sung exactly a fifth lower by another voice ( vox organalis ).
In contrast to the fourth organ , the distance between fifths was strictly adhered to in this form of music.
See also: Organum
Web links
http://www.uni-ulm.de/muz/publikationen/wk15/wk15.htm#Mehrstimmigkeit