There are just a few defining features that almost all fugues have in common. They begin with a single voice stating a strong musical idea. Two or more other voices subsequently present the same idea, while the voices that entered earlier continue with counterpoint. The original musical ideas are developed until the end of the piece. The exposition of the themes, or subjects, and their development comprises the fugue, which evolves through a fluid compositional process.
The structure outlined here is drawn from the architecture employed by J. S. Bach in both books of the Well-Tempered Clavier. Many fugues written after this exhibit similar structural characteristics. While each fugue is unique, there are shared characteristics. The three identifiable sections of most fugues and their components are defined below.
The subject (S) is stated in the tonic key. It contains one or more strongly recognizable motives. It usually begins on a member of the tonic triad and ends on a strong beat. The first voice states the subject alone and is joined by the other voices as they each state the subject. Many fugue subjects have two contrasting segments: a head and a tail. Two types of fugue subjects were identified in the 18th century. A soggetto subject is short and simple, compared to a flowing andamento subject, which is longer and contains several ideas.
The second voice enters with the subject, usually in the key of the dominant, in any octave. This statement of the subject is referred to as the answer (A). If it is an exact transposition of the subject, it is called a real answer. If an adjustment has been made to intervals in the subject to fit the harmony of the moment, it is called a tonal answer.
The first voice continues with melodic material as the second voice enters with the answer. If this material is thematic and used consistently to accompany each appearance of the subject, it is referred to as a countersubject (CS). It is written in invertible counterpoint, and used above or below appearances of the subject.
Entries of the subject in the tonic, along with answers in the dominant, continue to alternate until all voices have entered. If the exposition is unusually short, the composer may add extra, redundant, entries of the subject and answer. Once a voice has entered, it is not generally dropped from the exposition. After the initial statement of the subject, a composer may write a link or bridge that serves to continue rhythmic motion until reaching a favorable point for entry of another voice.
The end of the exposition occurs after all the voices have entered, usually on a strong cadence. The exposition may end with a small amount of closing material after the statements of the subject, similar to a short codetta. The cadence is normally in the key of the dominant if the piece is in a major key. If the piece is in a minor key, the cadence may occur on either the relative major or on the dominant minor tonality. A multiple fugue has more than one subject, each with its own exposition.
Following the exposition, a series of entries of the subject is distributed among the voices in related keys. This is sometimes referred to as the development section. The middle entries are separated by episodes. An episode consists of free material, usually containing motives drawn from the subject or countersubject. The most common technique applied in episodes is the sequencing of fragments. These might be inversions of the head or tail of the subject, repeated a few times, typically in a downward fifth progression.
Often a false entry, or mock entry appears in which an incomplete statement of the subject becomes episodic. Rests are used to let the voices breathe, and to draw attention to them as they re-enter the texture. A high point of maximum intensity sometimes occurs about three-fourths of the way through the fugue. This climax may consist of overlapping entries of the subject in different voices, a technique known as stretto.
A fugue typically concludes with one or more final statements of the subject in the tonic key. This is often an incomplete recapitulation of the exposition. The subjects may appear in stretto, or there may be a pedal point on the dominant. A brief coda, or codetta, may be used to lend a sense of finality to the conclusion of the fugue. A number of the fugues in the Well-Tempered Clavier have a short coda in a free recitative style.
Students should complete Assignment 1 on page 195 of the text. An audio recording of Fugue 6 from the WTC Book I on page 378 of the anthology is provided below. The questions asked in this assignment can be applied to the analysis of any fugue. Complete a Fugue Analysis Form for this, or any fugue you analyze.
J. S. Bach: Fugue from Well-Tempered Klavier, Book I, No. 6
The Counterpoint Course on hansenmedia.net contains thorough treatment of the Baroque Fugue, along with Inventions, Canons and Rounds, and Classical Period examples of fugal techniques. Here are links to that content:
Inventions & Sinfonias by J. S. Bach
The Canon & The Round
Classical Period Counterpoint