428. Here the key of the piece is A major; the subject, therefore, being throughout in the dominant, the answer is in the tonic (§ 70). The first answer is at the regular interval; but the next entry of the subject (bar 4) is in the key of D instead of E; and from this point the entries, though each is generally at a fourth or fifth above or below the preceding, are so irregular that it is impossible to say which are subjects, and which are answers. We have therefore marked them all with 'S.'
429. It must be remembered that it is not allowed to transpose any phrases of the canto fermo into other keys; consequently only such modulations are available as can be introduced without doing violence to the original form of the choral. The same rule, of course, applies to the variety of fugue noticed in the earlier part of this chapter. Any other modulations, if used at all, can only be employed in what may be termed the interludes between the different lines of the choral. For instance, in our last example is a transient modulation at bar 9 to the key of E minor, introduced between the first and second lines of the choral.
430. Though it is advisable, where practicable, to employ the theme of the fugue as a counterpoint against the choral, it is also allowed to introduce the latter during the episodes. This considerably lightens the composer's labours; for (as we have already seen in Chapter VII.), though the materials of the episodes should have some connection with the subject or countersubject of the fugue, considerable liberty of treatment is allowed; and it will be much easier to introduce the choral than if we are bound to make it combine with a given theme.
431. A fine example of this method of treatment is seen in Mendelssohn's third Organ Sonata. There is here a double fugue, each subject having a separate exposition before they are combined. We quoted the first subject in § 38 (d). After a regular four-voice exposition, Mendelssohn at the 17th bar of the fugue introduces on the pedals the first line of the choral "Aus tiefer Noth schrei 'ich zu Dir," as a fifth part. The subject is only suggested above the choral, a new entry taking place on the last notes of the line. The entries of the second and third lines of the choral (bars 22 and 28) are similarly treated. At bar 35 the exposition of the second subject (a figure of semiquavers) begins. After it has been carried through all the voices there is an episode (bars 43 to 47) against the latter half of which (from bar 45) the next line of the choral is heard. At bar 57 the two subjects are combined, and the choral is introduced in the pedals from time to time; but in no one place throughout the fugue is either of the subjects in its entirety ever combined with the choral.
432. Mendelssohn's organ sonatas are so well known and so accessible that we have contented ourselves with merely referring to this fugue instead of quoting from it, because no short extracts from it would have been of much assistance to the student. It
