Two or more voices running alongside are treated in their relation to one another. Here the simple contrapuntal exercise, which sets them neatly together, and enfolds their rhythms into each other, will soon cease to satisfy. The best-sounding contrapuntal relations are those in which the voices show some slight traces of imitation. From the lighter imitations rises the stricter canon,[76] and at last the exact and regular succession of canonic repetitions is reached. The ideal is then an exercise in which every note of every voice has its imitative relations, and in which the whole is so interconnected that not a stone can be removed without bringing about the fall of the whole structure. We see Bach himself at this work. He works more lightly and elegantly, or more heavily and massively, not by arbitrary choice, but according to his subject. He begins with a Prelude, in which a gentle interlacing of voices answers to his mood; imitations, arising as if of their own accord, are inserted here and there. Then there is the final gigue of a suite, in which a rapid 6/8 theme has to be treated, in spite of its rapidity, with a certain canonic severity, which gives solidity to the conclusion of the whole suite. Then again we have a slow movement of intertwining voices, such as is found after the first preluding bars of the F sharp minor and C minor Toccatas; the voices alternately take up the mournful themes, with no strict regularity of exact succession, wonderfully floating, according to a fixed purpose, and gently swimming in their harmonies, with no recognisable support, almost a fairy-like echo of mediæval counterpoint. Or, finally, there is a regular fugue, which stands there like a monument, with hardly a superfluous note; in regular succession the voices present the theme in single or double counterpoint, always alternately imitating at the fourth or fifth, with a never-failing reference to the tonic key; and near the close, where still greater closeness of imitation is demanded, proudly and stiffly vaulted over with diminutions or augmentations of the theme. Bach has left us clavier-works in which we see perfected the simplest form which the contrapuntal conception could assume in his hands. These are the fifteen “Inventions” and the fifteen “Symphonies,” which in the manuscript appear with the inscription, “Straightforward directions, by which the amateur of the clavier, but especially the one anxious to learn, is shown a plain method, not only of playing properly in two parts, but also, on further progress, to undertake three obbligato parts; at the same time also not only gaining good inventions, but also personally performing them, and, above all, attaining a cantabile style in playing, as well as a good foretaste of composition.” This curious preface ends, “Verfertiget von Joh. Seb. Bach, hochf. Anhalt-Cöthenischen Capell-meister, Anno Christi 1723.”