A fugue begins with an exposition in which all the voices present the subject in an orderly, standardized way. (The contrapuntal lines in fugues are referred to as voices, even when the fugue is written for instruments. We will refer to the four lines in our Bach fugue for keyboard as the soprano, alto, tenor, and bass.)

First, the subject is announced in the most prominent fashion possible: It enters in a single voice without any accompaniment, while the other voices wait. Any voice can begin, and any order of entry for the other voices is possible; in the first diagram below, we follow the order of the example on our recording (alto, then higher up for the soprano, then below the alto for the tenor, and finally, lowest of all, the bass). After leading off, voice 1 continues with new material of its own while the subject enters in voice 2. Next, the subject arrives in voice 3 — with 1 and 2 continuing in counterpoint with it (and with each other), using more new material, and so on. This section of a fugue, the exposition, is over when all the voices have stated the subject.

After the exposition, the subject enters at intervals; usually it is spaced out by passages of other music. It may come at the top of the texture (in the soprano), the bottom (bass), or half hidden away in the middle; see the diagram at the end of this page. Some of these later subject entries come in different keys. Although the modulations to these other keys may not be very obvious, without them the music would be dull and stodgy.

The passages of music separating the later subject entries are called episodes. They provide a contrast to the subject entries. This is true even though their motives are often derived from the subject; in such cases, the episodes present not the subject in full but fragments of it, and so they stand apart from subject entries. After the exposition, the form of a fugue falls into an alternating pattern: Episodes of various lengths come between subject entries in various voices and in various keys. Here is a diagram of a typical short fugue: