The system of classification of 3 and 4 voices fugues according to the order of voice entries in the expositions we propose summarizes all possible combinations using simple circles like the following ones.

For 4 voices fugues:

For 3 voices fugues:

How do they work? They all work in the same way. We'll use circle c1 as example:

Circle c1

Circle c1 resumes 8 possible combinations. Here is how we use it:

1. We can start in any voice (S = soprano, A = alto, T = tenor and B = bass).
2. Once we have selected a voice we can move along the circle in any direction.

The other circles

Circle c2

Circle c3

Circle c0

Nomenclature

We propose the following method for naming the various combinations:

1. We first identify the circle: c1
2. Followed by the voice: c1b means we will use the circle c1, beginning with the bass.
3. Finally the direction, + clockwise direction, - counterclockwise direction. For example: c1b+ means c1, beginning with the bass in clockwise direction.

See Classification of 3 and 4 voices fugues by J. S. Bach according to the order of voices entry in the exposition.