A fugue begins with an exposition. After the exposition the composer alternates between episodes and subject presentations. Each fugue has its own structure that changes according to the composers invention and needs.
The first section of the fugue is the exposition. The exposition begins with one of the voices presenting the subject or theme of the fugue. A second voice follows with the answer (we will discuss later what makes an answer different to the subject). The other voices continue presenting subjects and answers. When all voices have presented a subject or an answer the exposition ends. In a 3 voice fugue we would have subject, answer, subject. In a 4 voice fugue we would have subject, answer, subject, answer.
The subject is the fugue's theme. In this example from Fugue BWV 847 in C minor by Bach the alto starts with the subject:
Once the subject has been presented by the soprano the alto continues with the answer. In this fugue the subject starts on C while the answer starts on a G. The answer is the subject transported an ascending fifth or descending fourth:
This answer is a tonal answer. Why? Here we compare the subject (lower stave) and the answer (upper stave) and we see that the answer is not always a perfect filth above. Why? The harmony in the first 2 beats is a tonic chord, so using a D instead of a C would make a dissonant note. The composer following a tonal reason can make subtle changes in the transposition. That is way we call this answer a tonal answer:
The are also real answers. In the C Major Fugue BWV 846 the answer is the subject exactly transported:
Here we compare the answer (upper stave) with the subject:
The first voice continues with a counterpoint while the second voice plays the answer. The same thing happens when the third voice enters. These counterpoints are called countersubject when they are regularly used in the fugue. If the counterpoints are different each time they are called free counterpoint. In the C minor fugue we have 2 countersubjects. Here is the structure of the exposition:
|A||Subject||Countersubject 1||Countersubject 2|
Between the answer by the soprano answer and the subject by the bass, Bach adds almost two measures to prepare the entrance of the third voice (the bass). This transition receives several names: codetta, interlude, or even episode and is built using materials from the subject and the countersubject. Here is the complete exposition: