All that has been said relative to the history of music has been considered exclusively from a single point of view—that of tones combined vertically, in musical chords. As a matter of fact, however, this harmonic conception of music has arisen comparatively recently in musical history. The simultaneous combination of different tones came about incidentally to the combining of two or more horizontal series of tones, or melodies, in the practice of counterpoint. As counterpoint became more complex, the resulting simultaneous combinations became more complex, and the problems of harmony arose and were solved.

Turning now to the history of counterpoint as a distinct musical development, we can say that at every stage of increasing complexity of counterpoint, the rules governing choice of intervals grew out of the currently accepted, although sometimes unconscious, harmonic principles of the time. The rules were successively modified, therefore, with the developing progress of harmonic conceptions in successive epochs. Thus the so-called “free” counterpoint taught today differs from “strict” counterpoint, as strict counterpoint differs from still earlier practice.

If we consider the actual practice of Bach in the matter of counterpoint, we find that he made a distinct contribution to the history of counterpoint by using material which suggested harmony of a complexity not accepted before his time; and in doing so he modified the rules of counterpoint so as to assimilate these complexities into a consistent and logical system. As was natural, the fact that he brought innovations into the practice of counterpoint carried his work beyond the comprehension of his contemporaries, who failed to accept fully what he wrote, and took his organ-playing more seriously than his composition. It was only after a hundred years that Mendelssohn's admiration caused Bach's work to be seriously studied as a significant contribution to the development of counterpoint.