Fugue

A contrapuntal composition built from a principal subject introduced in successive voices and developed through interweaving lines. Its form is flexible rather than fixed.

Subject (music): A principal musical idea that organizes a composition or movement. The subject supplies the recognizable material each voice introduces and transforms.
Counterpoint: The combination of independent melodic lines into a coherent musical texture. Fugue is a highly organized form of counterpoint, with voices sharing thematic material.
The Art of Fugue: Johann Sebastian Bach's unfinished collection of contrapuntal works based on one subject. Bach explores an unusually wide range of fugal transformations from a single theme.
Canon (music): A composition in which voices reproduce a melody at fixed intervals or delays according to a rule. Canon relies on sustained rule-governed imitation, whereas fugue permits more varied development.
Imitative counterpoint: Counterpoint in which a musical idea passes successively among independent voices. The fugue grew from traditions that made recurring imitation a compositional principle.
Answer (music): A restatement of a fugue subject, usually beginning in a related pitch area. The answer follows the opening subject, often adjusted to preserve tonal coherence.
Imitation (music): The restatement of a musical idea in one voice after another voice has presented it. Successive imitation is the basic process behind a fugue's subject entries.
The Well-Tempered Clavier: Johann Sebastian Bach's two-book collection of preludes and fugues in all major and minor keys. Its fugues offer a systematic survey of the form across all twenty-four keys.
Invention (music): A short contrapuntal keyboard composition that develops a musical idea through imitation. Inventions use imitative counterpoint but are generally less formally expansive than fugues.
Canzona: An instrumental form derived from vocal models, often organized into contrasting sections with imitative writing. Canzona practice contributed sectional and imitative techniques to early fugue traditions.

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