The works of many of the ninth century theorists such as Aurelian of Réomé, and Remy of Auxerre, suggest that some form of part-singing was practised in their day, though they leave us in confusion owing to the ambiguity of their language. The famous scholar Scotus Erigena (880) mentions organum, but in a passage that is difficult and obscure. Regino, abbot of Prum in 892, is the first to define consonance and dissonance in such a way as to leave no doubt that he considers them from the point of view of polyphony, that is to say, as sounds that are the result of two different notes sung simultaneously. In the works of Hucbald of St. Amand in Flanders, quite at the end of the century, if not well into the tenth (Hucbald died in 930 or 932, over ninety years of age), there is at last a definite and clear description of organum. The word organum is an adaptation of the name of the instrument on which the art could be imitated, or, perhaps, from which it partly originated, the organ; just as the Greeks coined a word from magadis. Of Hucbald’s life little is known save that he was born about 840, that he was a monk, a poet, and a musician, a disciple of St. Remy of Auxerre and a friend of St. Odo of Cluny. Up to within recent years several important works on music were attributed to him, of which only one seems now to be actually his--the tract, _De Harmonica Institutione_, of which several copies are in existence. This and the _Musica Enchiriadis_ of his friend St. Odo are responsible for the widespread belief that polyphony actually sprang from a hideous progression of empty fourths and fifths. Both theorists, in their efforts to confine the current form of extemporized descant in the strict bounds of theory, reduced it thus: to a given melody taken from the plain-song of the church the descanter or organizer added another at the interval of a fifth or fourth below, which followed the first melody or _cantus firmus_ note by note in strictly parallel movement. The fourth seems to have been regarded as the pleasanter of the intervals, though, as we shall see, it led composers into difficulties, to overcome which Hucbald himself proposed a relaxation of the stiff parallel movement between the parts. In the strict organum or _diaphony_ the movement was thus: [Illustration: Music] Either or both of the parts might be doubled at the octave, in which case the diaphony was called composite.