Various kinds of organum soon came into vogue. Three-part organum resulted from doubling the lower of the two parts an octave higher, and four-part organum from adding to these three parts the original upper part an octave lower, thus producing simultaneously moving octaves, fourths, and fifths. Such a progression of parts, quite obnoxious to ears accustomed to harmony, impressed Hucbald as ‘a delightful concord.’[6] As the experiments increased, the accompanying voice (the discant) was added above as well as below the chant (the _cantus firmus_, or fixed voice). The monotony of exclusive parallelism was broken by sometimes sustaining the same tone in one part while the other part moved up or down (oblique motion) or by letting the two parts move in contrary direction, and lastly, by mixing these three kinds of tone movement, thus producing greater variety in the intervals used. When this freer movement of parts was recognized as essential to more pleasing vocal effects, the word discant came to be applied to it to distinguish it from the more primitive form of movement--organum--in parallel fourths, fifths, and octaves. Until the thirteenth century the intervals most used in all styles of part-writing were fourths, fifths, octaves, and unisons. Thirds and sixths, though occasionally permitted, were regarded as dissonances until the period when harmony came to be a conscious element of musical thought. [Illustration: The Playing Angels] _Altar piece by Hubert and Jan van Eyck_ Until a definite system of notation was devised, the discanting parts to the chants were extemporized by the singers. But when the staff was invented and notes or points were employed to indicate the exact pitch of the tones of the melodies, the name counterpoint (_punctus contra punctum_, note against note) was given to the part or parts added to the chant (_cantus firmus_). The term counterpoint[7] displaced discant in the thirteenth century, and from this time the art of counterpoint developed as the number of added parts increased and the various kinds of intervallic relationships among the interdependent parts were recognized and systematized.