The general introduction of the Gregorian chant established the melodic basis of modern music. It will be well for the reader to bear in mind that the three elementary constituents of music are melody, harmony, and rhythm. Melody is produced by the successive sounding of single tones of different pitch. Harmony is produced by the simultaneous sounding of single notes of different pitch. When those notes are united according to rule we call the result a chord. Rhythm is the regular recurrence of long and short beats. Now, if the reader has comprehended the account given of the early Roman chant, he will perceive that it embodied only one of the elements of music, namely, melody. There was no harmony, because everything was sung in one part. It was simply a plain chant; and when the organ first came into use to accompany it, the instrument played the same succession of single notes as the voices sang. This fact must be kept clearly in mind in order to understand the next steps in the development of modern music. In the beginning there was only melody; and that was like the earth before the creation, without form and void. There was no musical rhythm in either the Ambrosian chant, which followed the prosodial quantity of the words, nor in the Gregorian, which did not follow it. The text was prose; consequently it did not have a regular recurrence of long and short syllables, as poetry does, and therefore the music, following the text, could not have rhythm. All that existed in the beginning of the modern tone art was the raising and lowering of the voice through a certain number of intervals. How harmony and rhythm made their appearance in the early stages of the art, and what forms they took, must next be related. Chapter II Harmony, Notation, and Measure The Organum of Hucbald--Use of combinations disagreeable to modern ears--Appearance of rhythm--Work of Franco, of Cologne--Establishment of Dual and Triple Measure--Introduction of notes to represent sounds of different duration.