|Op 87 - No. 7 - A major - Fugue|
Thank goodness for the answer. Remember, one of the only strict rules of the fugue is that it must begin with an exposition where every voice states the subject. After the first voice states the unaccompanied subject, the second voice enters with what is called the answer. It is, of course, an imitation of the subject, but with at least one crucial modification: it starts on a different pitch, most often, a perfect 5th higher or a perfect 4th lower. This is another strict rule of fugal composition and it is a great idea: it imbues the exposition with a fundamental harmonic tension and complexity which is often absent from other contrapuntal forms such as invention and canon. Since most fugues have more than 2 voices, this rule is applied again and again to each subsequent subject entry in the exposition. The 3rd voice provides the answer to the answer, again, a 5th up or 4th down, that is, the original subject again. In the fugue with 4 voices, the exposition will be subject, answer, subject, answer.
In the case of this fugue, the contrast of subject and answer provides the essential harmonic shift necessary to put things in motion. Instead of an implied harmonic progression within the subject itself, the harmonic progression is implied by the movement from subject to answer, across multiple subject entries. As the subject adheres to the A major chord, so the answer (a real answer) adheres to the E major chord.
What is even more astonishing about this fugue, is that it uses two countersubjects, both of which are themselves restricted to triad tones! As the first answer states the E major chord, so does the first appearance of the first countersubject. As the third voice enters with the original subject again, the subject, the countersubject and the first appearance of the second countersubject are all stating an A major chord.
So how does Shostakovich make this fugue become more than just a static series of tonic and dominant chords as in one long perfect cadence?
Once beyond the exposition, all fugues tend to move around to different keys, to modulate. This is chiefly accomplished by episodes that prepare the context for the next subject to enter in a new key. Of course, the episodes may in themselves provide musical interest and typically do. In addition, the strict tonal relationship between subject and answer need not be maintained beyond the exposition, whether or not the fugue modulates. Here is the secret to this fugue: compelling episodes, modulations into keys other than A major (or E major), and answers starting on pitches other than the 5th. This is most dramatically apparent in two places. The 4th subject entry appears in a minor key, F sharp minor, the relative minor of A major (its answer is naturally C sharp minor). One of the final subject entries provides a welcome touch of warmth by providing a D major chord, rooted on the forth rather than the fifth of the A major scale. It is in fact part of the final sequence of entries that create a large scale perfect cadence of tonic, subdominant, dominant, tonic, or, I, IV, V, I. (A, D, E, A).
Note: while the majority of fugues have their answers begin a 5th away from the subject's starting tone (a perfect 5th above or a perfect 4th below), this is not the only possibility. Because of the difference between real and tonal answers and the fact that some subjects modulate, the answer might well begin on an alternate pitch. Even so, the essential concept of movement by a 5th between subject and answer is still intended. There are examples of modern fugues (e.g. by Hindemith) that place the answer a 3rd away from the subject representing a fresh and different approach.