We may plunge at once to the very foundation, the harmonic groundwork. As we have seen, perhaps the most important accomplishment in music of the seventeenth century was the discovery and establishment of key relationships in that harmonic conception of music which has endured almost to the present day. Instrumental forms developed upon this re-organization of musical material. Subsequently, however polyphonic the texture of a piece of music--a fugue of Bach’s, for instance--might be, its shape was moulded upon a frame of harmony. The piece was in a certain key, clearly affirmed at the beginning and at the end, points in the structure which in a piece of music as in a paragraph are naturally the most emphatic. Within these limits there was the life and variety of a harmonic development, which, departing from the tonic key, must return thence. Long before the year 1700 the regulation of such harmonic procedures had definitely fixed the symmetrical plan of two forms: the so-called aria form and the binary form. Neither was in itself capable of much development; and it was in a sort of fusion of both that the harmonic plan of the triplex form was created. The aria form was in three sections which we have elsewhere represented by the letters A, B, A. A, the opening section, was all in the tonic key, and was practically complete in itself. B, the second section, was in a contrasting key or was harmonically unstable. A, the third section, was but an exact repetition of the first, to give balance and unity to the whole. The limitations of the form were essentially harmonic. The first section offered little or no chance for modulation. Its tonality must be unmistakably and impressively tonic. Therefore it did not develop into the second section by means of harmonic unrest. The second was simply a block of contrasting harmonies, like a block of porphyry set beside a block of marble. Frequently, however, the second section was incomplete without the third. In such cases a hyphen between the B and the second A in our lettered scheme would represent the relations between the three sections more nearly, thus: A, B-A. The binary form, in which most of the dance movements of the suite were composed, was usually shorter than the aria form; but though apparently simpler, it was, from the point of view of harmony, more highly organized. It consisted, as we have seen, of two sections, each of which was repeated in turn. The first modulated from the tonic key to the dominant or relative major; the second from that key back to the tonic again. It will be observed that the first section really grew into the second by harmonic impulse; for the first section, ending as it did in a key that was not the key of the piece, was incomplete. The two sections together not only established a perfect balance of form and harmony, but had an organic harmonic life which was lacking in the aria.