Double fugue
Fugue with two independent subjects.
What it means
A double fugue is a fugue that is based on two independent subjects instead of one. Both subjects are treated fugally, that is, introduced one after another in the voices, and combined with each other in the course of the piece. Accordingly, with three subjects one speaks of a triple fugue.
Musical use
Two types of construction are to be distinguished. In the first, both subjects are introduced together from the start: one sounds together with the other, which serves as a fixed counterpart, and both wander together through the voices. In the second, the first subject is worked through first in a complete exposition, then the second in one of its own, and only in the later course do both appear together — often as the climax of the movement.
The prerequisite in every case is that the subjects stand in double counterpoint to each other: they must be able to exchange their position, so that each can sound above and below the other without the writing becoming faulty. So that the two subjects remain distinguishable, they are usually built clearly differently — for instance a slow, striding one against a moving, figured one.
The double fugue is to be distinguished from a fugue with a retained countersubject: there too a constant counter-voice accompanies the subject, but it is not introduced fugally in its own right. The boundary is not always sharp, and the designations are handled with varying strictness in the literature. To be distinguished as well is the counter-fugue, whose mark is the inversion of the subject in the answer.
As for the layout, one distinguishes two ways: either both subjects are exposed together from the start, or each first receives a development of its own before they are combined. The prerequisite in every case is that the subjects stand in double counterpoint, that is, work in both positions in relation to each other. The triple fugue extends the principle to three subjects.
Performance practice
First make clear to yourself which type of construction is present, and mark the entries of both subjects in different colours. Shape the subjects differently so that they remain distinguishable in the sonority as well. And give the place where both first meet its weight.