All modern music differs from the ancient in two important particulars--_Harmony and Tonality_. Harmony is the use of combined sounds. These may be either dissonant, inharmonious in relation to each other, or harmonious, agreeable. All points of repose in a harmonized piece of music must be consonant; or, to say it differently, the combined sound (chord) standing at the beginning or end of a musical phrase must be harmonious. All the elements in it must bear consonant relations to all the others. Between the points of repose the combined sounds may or may not be consonant. Under certain conditions dissonances make an effect even better than consonance--better because more appealing. The law of the introduction of dissonances is that every dissonance must arise out of a consonance, and subside into a consonance. When this law is observed there is hardly any combination possible in the range of music which may not be employed with good effect. Here already we have a progress in perception of tones, in the ability to discriminate between those which harmonize and those which dissonate. All consonance and dissonance are purely relative. There is no such thing as a dissonant tone in music, by itself considered; a tone becomes dissonant by being brought into juxtaposition with some other tone with which it does not agree. This part of the development of a tonal sense had its beginnings in Greece, but only reached the point where the most elementary relations were regarded as agreeable. The octave, the fourth and the fifth, were the only consonances which they knew, and of these they used in the combined sounds of their music only the octave. The third, which with us is the most agreeable part of a pure harmony, because it adds so many elements of agreement to the combined sound into which it enters, was not only regarded as a dissonance by them, but actually _was_ a dissonance as they tuned their scale.