Fauxbourdon
Historically and properly, it is a 15th-century French technique of composition
in which a plainsong melody transposed
to the upper octave is notated together with a contrapuntal
part moving along at the lower sixth or occasionally at the octave,
while a middle part is extemporized by a singer doubling
the melody at the lower fourth throughout.
In modern usage, a general designation for harmonic progressions based
on parallel sixth chords such as occur not
only in old music but in the works of Bach,
Beethoven,
and others.