By the beginning of the thirteenth century the science of music had reached the point where music could be accurately notated as regards both pitch and time relationships and its further development became correspondingly accelerated. The organization of music on the twofold basis of regularity of stress or accent and of fixed proportions in the division of time-units was hastened by the growing desire of singers to add a new voice-part to the old Gregorian chant. This practice of part-singing, at first called ‘organum,’ later ‘discant,’ undoubtedly had its origin in the study-rooms of the choirs and singing schools. The choristers were naturally chosen because of their unusual aptitude for music. The larger part of their time was given up not only to the perfecting of means for the most effective performance of the church music, but also to the study of the theory and practice of music in all its then known phases. The creative instinct more and more seized upon them. Notwithstanding ecclesiastical restrictions the singers were too much under the seductive spell of the inner spirit of their art not to yield to the ready temptation of delving into the infinite possibilities of new tonal combinations and devices that lay so close at hand. When the idea of singing two melodies at the same time was once grasped (we have no definite knowledge how it was first suggested), the singers took it up with avidity. At first experiments were restricted to two voices or parts. While one chorister was singing a familiar chant-melody another would sing a second melody an octave or a fourth or a fifth below it, usually joining it at the end in unison. The progression of two voices or parts moving in parallel octaves was known to the Greeks and was called by them ‘magadizing’--from the magadis, a stringed instrument. The singing of two concurrent parts in parallel fourths or fifths did not offend mediæval ears as it does modern ears, probably because of the exact parallelism of such melodic movement, which is merely a different kind of unison.[5] The earliest parallel movement was evidently in fourths, not in fifths, as usually stated in musical histories. (See Weinmann, ‘History of Church Music,’ page 74.)