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415. In addition to the different varieties of fugues treated of in the preceding chapters, we not infrequently find fugues written upon a choral. As with a canon on a choral (Double Counterpoint, § 391), there are two ways in which this can be done. We can either take the melodies of the choral itself as the subjects of our fugue, or we can write a fugue on an entirely independent subject, introducing the choral in longer notes in one of the voices as a kind of canto fermo. Owing to the much greater freedom of its form, it is far easier to write a fugue on a choral than a canon.

416. We will first speak of that form in which the choral itself furnishes the subjects of the fugue. The plan most often followed is to write a separate fugal exposition for each line of the choral, the last entering voice, which, for the sake of clearness, is generally one of the outer parts, introducing the melody, in most cases, though not invariably, by augmentation.

417. As this method will be best taught by examples, we give two entire fugues of this kind, one very simple, the other rather more elaborate. The first is by one of Bach's great predecessors, the Danish organist Buxtehude. It is on the choral "Erhalt uns, Herr, bei deinem Wort." In the original there is only one flat in the signature, according to the custom of the time, though the key of the piece is G minor; for the convenience of students, we adopt the modern key-signature.