For instance, in one "Alleluia" ([//upload.wikimedia.org/wikipedia/commons/3/34/Perotin_Alleluya_Quarten_for_wikipedia.mid Listen]) by Pérotin, the fourth is favoured. Elsewhere, in parallel "organum" at the fourth, the upper line would be accompanied a fourth below. Also important was the practice of "Fauxbourdon", which is a three voice technique (not infrequently improvisatory) in which the two lower voices proceed parallel to the upper voice at a fourth and sixth below. "Fauxbourdon", while making extensive use of fourths, is also an important step towards the later triadic harmony of tonality, as it may be seen as a first inversion (or 6/3) triad.