(7) Every mutation should be made in approaching or quitting the tonic or dominant.
The counter-subject is primarily to be regarded as an accompaniment to the subject or answer. But it is more than this, for it ought to be made so melodious as to be an available foil to the subject when used in alternation with it, or with the answer. It should also be, in most cases, so constructed as to work in double counterpoint with the subject. It usually makes its first appearance as an accompaniment to the first entry of the answer, after the subject has been duly announced by itself. We now proceed to give an example of the commencement of a fugue, containing subject, answer, and counter-subject. Such a commencement is called 'the Exposition.'
When the countersubject is introduced simultaneously with the subject at the beginning of a fugue, it should be looked on rather as a second subject, and treated strictly as such throughout the fugue. In such a case the piece would be properly described as a Double fugue, or Fugue with two subjects. Similarly there are fugues with three or more subjects; the only limitation being that there should always be fewer subjects than parts; though there are exceptions to this rule, as e.g. 'Let old Timotheus' in Handel's 'Alexander's Feast,' where there are four subjects and only four voice-parts.
It is very often desirable to interpose a few notes to connect the subject and answer, and to facilitate the necessary modulations from tonic to dominant, and back again. Such connecting notes are named the Codetta, conduit, or copula, and are very useful in rendering the fugue less dry and cramped.
The following is the exposition of a two-part fugue, including a codetta:—
After the exposition is completed by the successive and regular entry of every part, it is well to make use of fragments of the materials already announced, working them up contrapuntally into passages of imitation, and modulating into nearly related keys for a few bars, before returning again to the subject and answer. These may then be introduced in various kindred keys, according to the taste of the composer, so as to secure variety and contrast, without wandering too far from the original key of the piece. As the fugue goes on, it is important to keep the interest of it from flagging by the introduction of new imitations, formed of fragments of the original materials. These passages are termed Episodes. With the same object in view it is customary to bring the subject and answer nearer to one another as the fugue draws towards its conclusion. The way to effect this is to make the entries overlap; and this is called the Stretto (from stringere, 'to bind'). Thus the above subject would furnish a stretto as follows:—
Some subjects will furnish more than one stretto. In such cases the closest should be reserved for the last. [Stretto.]
But there are many other devices by which variety can be secured in the construction of a fugue. For the subject can sometimes be inverted, augmented, or diminished. Or recourse may be had to counterpoint at the 10th or 12th. The inversion of the above subject would be as follows—