The aria form is one of the simplest in music, one of the most effective as well, and was the first to develop under the influence of the Italian opera of the seventeenth century. It has frequently been called the A-B-A form. This is because it is made up of three distinct sections of which the first and last, predominantly in the tonic key, are identical, and the middle in some contrasting key or keys and of contrasting musical material. To spare themselves the trouble of writing out the last section, composers adopted the convention of merely writing the Italian words _da capo_ (from the beginning) at the end of the second section, and of placing a double bar at the end of the first, over which the singer or player was not to pass upon his second performance of this section. Bach could have adopted this economical device, had he so desired, in the four preludes just mentioned; for each of them proves, upon examination, to be composed of three distinct sections, the middle more or less the longest, the first and last note for note the same. We have already remarked how most of Bach’s fugues, especially the shorter ones, can be divided into three sections based upon harmony. In the preludes to the English suites the question of musical material enters into the division. Take for analysis the prelude in A minor to the second suite. The first section ends at the beginning of the fifty-fifth measure. It will be seen to open with a bold figure, the first notes of which are at once imitated in the left-hand part. There follows then a constant flow of figure work over a relatively simple harmonic foundation and through orderly sequences, the hands frequently imitating each other. Fragments of the opening phrase are heard five times. In the thirty-first measure a very distinct phrase is introduced, still in the tonic key, it will be observed, though in dominant harmony; and this is repeated in purely conventional manner in three registers, giving way to formal passage work which, falling and rising, leads to a good stout reiteration of the opening motive. With this the first section ends, in a full tonic cadence. The second section begins at once with a wholly new figure which dominates the music from now on up to the one hundred and tenth measure. At this measure the second section ends, and here Bach might have written the words _da capo_; for what follows is but a repetition of the first fifty-five measures. It must be noticed that, although the middle section is decidedly dominated by a figure which does not appear in the first, still the first theme is not allowed to be forgotten. It may be found five times in the course of the middle section, dividing, as it were, the new material into distinct clauses, and serving as well to impress upon our ears the unity of the piece as a whole.