The entire course of harmonic perception in modern music may be roughly divided into three steps: First, the recognition of consonance, especially of the most fruitful consonance of all--that of the thirds, and the differentiation between consonance and dissonance. A second step involved the recognition of dissonance as an element in musical expression, on account of the motion it imparts to a harmonic movement. Third, the establishment of these materials of music in the mind in such depth and fullness that their aesthetic implications became realized as elements of expression, so that when a composer had a certain feeling to express, the proper combination of consonance and dissonance immediately presented itself to his mind. The first of these steps was taken by the minstrels of the north, somewhere between the Christian era and the tenth century. The second was the particular work of the old French school, the Netherlanders, and of all who composed music between about 1100 A.D. and the epoch of Palestrina, about 1600. The third, the spontaneous application of musical material to the expression of feeling, had in it another element, that of tonality, concerning which it is proper to say something at this point. By "tonality" is meant the dependence or interdependence of all the tones in a key upon some one principal tone called the Key-tone. The tonality of the music of the ancients was wholly artificial and unreal. A mode and a point of repose for the melody were chosen arbitrarily; the beginning was here made, and still more the ending was conducted to this point of repose. Between the beginning and the ending the same tones were employed, whether the melody proposed to repose upon re, upon fa or do. The usual points of repose in Greek music were mi, fa and re; never upon do, the real key tone, and rarely upon la, the natural tonic of the minor mode.