Fauxbourdon

Fauxbourdon (also Fauxbordon, and also commonly two words: Faux bourdon or Faulx bourdon) – French for false bass – is a technique of musical harmonisation used in the late Middle Ages and early Renaissance, particularly by composers of the Burgundian School. Guillaume Dufay was a prominent practitioner of the form (as was John Dunstaple), and may have been its inventor. The homophony and mostly parallel harmony allows the text of the mostly liturgical lyrics to be understood clearly.

In its simplest form, fauxbourdon consists of the cantus firmus and two other parts a sixth and a perfect fourth below. To prevent monotony, or create a cadence, the lowest voice sometimes jumps down to the octave, and any of the accompanying voices may have minor embellishments. Usually just a small part of a composition employs the fauxbourdon technique.

In a hymn, the term is sometimes used when the sings in parallel octaves, with some singers singing a treble descant over the melody, but the term was historically used to indicate an arrangement of the tune in four parts with the melody in the tenor voice, such as those composed by sixteenth- and seventeenth-century English composers including John Dowland, Giles Farnaby, and Thomas Ravenscroft.

...
Wikipedia