Here the answer ends on the third of the dominant; a codetta is therefore introduced (§ 193) to lead back naturally to the key of the tonic, in which the subject reappears. The codetta is made from a modified form of the first notes of the subject treated sequentially, and accompanied (also sequentially) by the first notes of the countersubject in inverse movement. This is not an episode, because the exposition is not yet complete. The subject then enters in the bass, with the countersubject above it. No additional entry being here required, as the subject was announced by the middle voice (§ 186); the exposition ends here, and is followed at once by the first episode.
216. If we examine this episode, we shall see that it is made from the same material as the codetta, but with different treatment. The two upper voices have a theme founded on the first notes of the subject, the alto imitating the treble as a canon in the fifth below. The bass gives the commencement of the countersubject, not (as in the codetta) by inverse movement, but in its direct form. Observe also how, by means of sequence, a modulation is effected to the key of the relative major, in which key the subject follows in the treble voice immediately on the conclusion of the above extract.
217. We said in Double Counterpoint (§ 307) that imitation was a most important ingredient in fugue, and the quotation just given will show how it is to be used. Except in a stretto, the construction of which will be explained in our next chapter, imitation is seldom found during the entries of the subject itself; but it is almost constantly employed in the episodes. By this means unity of character is given to this part of the work, and anything like patchwork is avoided.
