The marked dissimilarity between the music of the mediaeval school and that of the present era is to a large extent explained by the differences between the key and harmonic systems upon which they are severally based. In the modern system the relationship of notes to the antithetic tone-centres of tonic and dominant, and the freedom of modulation from one key to another by means of the introduction of notes that do not exist in the first, give opportunities for effect which are not obtainable in music based upon the Gregorian modes, for the reason that these modes do not differ in the notes employed (since they include only the notes represented by the white keys of the pianoforte plus the B flat), but only in the relation of the intervals to the note which forms the keynote or “final.” The concoction of music based on the latter system is, strictly speaking, melodic, not harmonic in the modern technical sense, and the resulting combinations of sounds are not conceived as chords built upon a certain tone taken as a fundamental, but rather as consequences of the conjunction of horizontally moving series of single notes. The harmony, therefore, seems both vague and monotonous to the ear trained in accordance with the laws of modern music, because, in addition to being almost purely diatonic, it lacks the stable pivotal points which give symmetry, contrast, and cohesion to modern tone structure. The old system admits chromatic changes but sparingly, chiefly in order to provide a leading tone in a cadence, or to obviate an objectionable melodic interval. Consequently there is little of what we should call variety or positive color quality. There is no pronounced leading melody to which the other parts are subordinate. The theme consists of a few chant-like notes, speedily taken up by one voice after another under control of the principle of “imitation.” For the same reasons the succession of phrases, periods, and sections which constitutes the architectonic principle of form in modern music does not appear. Even in the “familiar style,” in which the parts move together like blocks of chords of equal length, the implied principle is melodic in all the voices, not tune above and accompaniment beneath; and the progression is not guided by the necessity of revolving about mutually supporting tone-centres.