The alternation of the Subject with the Answer—called its Repercussion (Lat. Repercussio; Ital. Repercussione; Germ. Wiederschlag) is governed by necessary, though somewhat elastic laws. Albrechtsberger gives twenty-four different schemes for a Fugue in four Parts only, showing the various order in which the Voices may consistently enter, one after the other. The great desideratum is, that the Answer should follow the Subject, directly; and be followed, in its turn, by an immediate repetition of the Subject, in some other Part: the process being continued, until all the Parts have entered, in turn, with Subject and Counter-Subject, alternately, and thus become entitled to continue, for a time, as Free Parts. But the regularity of this alternation is not always possible, in Choral Fugues, the management of which must necessarily conform to the compass of the Voices employed. For instance, in Brahms's 'Deutsche Requiem,' there are two Subjects, each embracing a range of no less than eleven notes—a fatal hindrance to orthodox fugal management.
When the Subject has been thus clearly set forth, so as to form what is called the Exposition of the Fugue, the order of its Repercussion may be reversed; the Answer being assigned to the Parts which began with the Subject, and vice versâ: after which the Fugue may modulate at pleasure. But, in common language, the term Subject is always applied, whether accurately or not, to the transposed Theme, even though it may appear in the aspect proper to the Answer.
As the Fugue proceeds, the alternation of Subject and Answer is frequently interrupted by Episodes (Ital. Andamenti; Fr. Divertissements), founded on fragments of the Subject, or its Counter-Subjects, broken up, in the manner explained on page 135; on fragments of contrapuntal passages, already presented, or on passages naturally suggested by these. Great freedom is permitted in these accessory sections of the Fugue, during the continuance of which almost all the Parts may be considered as Free, to a certain extent. Nevertheless, the great Fuguists are always most careful to introduce no irrelevant idea into their Compositions; and every idea not naturally suggested by the Subject, or by the contrapuntal matter with which it is treated, must necessarily be irrelevant. It is indeed neither possible nor desirable, that every Part should be continuously occupied by the Subject. When it has proposed this, or the Answer, or one of the Counter-Subjects deduced from them, it may proceed in Single or Double Counterpoint with some other Part. But, after a long rest, it must always re-enter with the Subject, or a Counter-Subject; or, at least, with a contrapuntal fragment with which one or the other of them has been previously accompanied, and which may, therefore, be fairly said to have been suggested by the Subject, in the first instance. And thus it is, that even the Episodes introduced into a really good Fugue form consistent elements of the argument it sets forth. In no Fugue of the highest order is a Part ever permitted to enter, without having something important to say.
After the Exposition has been fully carried out, either with or without the introduction of Episodes, the subsequent conduct of the Fugue depends more on the imagination of the Composer than on any very stringent rule of construction; though the great Fuguists have always arranged their plans in accordance with certain well-recognised devices, which are universally regarded as common property, even when traceable to known Masters. And here it is that the ingenious Devices (Fr. Artifices; Germ. Kunsteleien) described at page 135 as accessory elements of the Fugue, are first seriously called into play. The Composer may modulate at will, though only to the Attendant Keys of the Scale in which his Subject stands. He may present his Subject, or Counter-Subject, upside-down—i.e. inverted by Contrary Motion; or backwards, in 'Imitatio cancrizans'; or, 'Per recte et retro'—half running one way, and half the other; or, by single or double Augmentation, in notes twice, or four times, as long as those in the original; or by Diminution, in notes half the length. Or, he may introduce a new Counter-Subject, or even a Canto fermo. In short, he may exercise his ingenuity in any way most congenial to his taste, provided only that he never forgets his Subject. The only thing to be desired is, that the Artifices should be well chosen: not only suggested by the Subject, but in close accordance with its character and meaning. It is quite possible to introduce too many Devices; and the Fugue then becomes a mere dry exhibition of learning and ingenuity. But the Great Masters never fall into this error. Being themselves intensely interested in the progress of their work, they never fail to interest the listener. Among the most elaborate Fugues on record are those in Sebastian Bach's 'Art of Fugue,' in which the Subject given on page 136 is treated with truly marvellous ingenuity and erudition. Yet, even these are in some respects surpassed by the 'Et vitam venturi,' which forms the conclusion of Cherubini's Credo, Alla Cappella, for eight Voices, in Double Choir, with a Thorough-Bass. The Subject (quoted on page 136) is developed by the aid of five distinct Counter-Subjects, three of which enter simultaneously with the Subject itself; the First after a Minim-rest; the Second after three Minims; the Third after two bars: the Subject itself occupying three bars and one note of Alla Breve Time. It may therefore justly be called a Quadruple Fugue. The two remaining Counter-Subjects enter at the fifth and sixth bars, respectively; and, because the first proposal of the Subject comes to an end before their appearance, Cherubini, though giving them the title of Counter-Subjects, does not number them, as he did the
