Although this system of six modes was recognized by medieval theorists, in practice only the first three and fifth patterns were commonly used, with the first mode being by far the most frequent . The fourth mode is rarely encountered, an exception being the second clausula of "Lux magna" in MS Wolfenbüttel 677, fol. 44 . The fifth mode normally occurs in groups of three and is used only in the lowest voice (or tenor), whereas the sixth mode is most often found in an upper part .