A polyphonic keyboard composition in two or three voices exploring imitative counterpoint featuring one or more recurrent fugues known as motives.

The term invention is unique and somewhat peculiar to Bach and a few of his contemporaries. Bach wrote two different sets of what are today formally known as his inventions: the first, a set of 15 two-part inventions, the second, a set of 15 three-part inventions which were actually titled sinfonia, not inventions. Due to their excellent artistic quality and a general commonality to their construction, the inventions have established a sort of genre or type of piece, at least within the vast catalog of Bach's own work. Many other compositions in his keyboard repertoire are regarded as inventions though they lack the explicit title. Of particular interest here are several of the preludes in the Well-Tempered Clavier (see below). In addition, the invention is closely related to the fugue and provides an interesting comparison and contrast as an alternate type of imitative counterpoint.

In the most general sense, an invention is like a fugue in that it is a polyphonic composition for a fixed number of voices featuring a basic recurring theme (like a subject), counterpoint, episodes, a common set of transformations (e.g. inversion, augmentation, etc.), and the general artistic goals of creating a compelling narrative. But an invention contrasts strongly with a fugue in a number of key aspects.

The recurring theme of an invention is more properly called a motive rather than a subject. Recurring counterpoint is called counter motive rather than countersubject. Compared to the subject of a fugue, the motive of an invention is brief, more rhythmic than melodic, less rhythmically varied with a more simple implied harmonic background. An invention lacks the formal exposition of a fugue: multiple voices are immediately active featuring motive and counterpoint from the start. Perhaps most important is that the imitative answer of the motive in an alternate voice occurs at the octave, not the fifth of the scale: the invention lacks a harmonic tension and complexity fundamental to the fugue. Since inventions are written for no more that three voices, half of them for only two, their texture is much less dense than the fugues of three, four or even five voices. Inventions are generally shorter, lighter, looser and fundamentally "easier" that most fugues. Evidence to this is that invention-like compositions are suitable as preludes to fugues in the Well-Tempered Clavier. Like many of the preludes in general, inventions provide wonderful opportunities to experience polyphony which is a bit less demanding than most formal fugues.

Like the set of fugues in the Well-Tempered Clavier, the sets of two and three-part inventions provide a wonderful catalog of variety, diversity, ingenuity, personality and artistic excellence within the specific constraints of a single type of composition. At least a few of Bach's two-part inventions are well-known to beginning piano students, general media consumers and, sadly, cell-phone users. Perhaps you will recognize them. Here are their primary motives:

Two-part Invention No. 4 D minor

Two-part Invention No. 8 F major

Many scholars classify several of the preludes in both books of the Well-Tempered Clavier as inventions, certainly in spirit and style if not literally in the mold of the two formal sets. They are collected here for your exploration. To approach music with an appropriate conceptual template pattern is often a way to grasp the narrative of the music more quickly, make it familiar, and, paradoxically, to rapidly discover its unique character.

Book 1, Prelude No. 3, C sharp major
Book 1, Prelude No. 4, C sharp minor
Book 1, Prelude No. 9, E major
Book 1, Prelude No. 11, F major
Book 1, Prelude No. 12, F minor
Book 1, Prelude No. 13, F sharp major
Book 1, Prelude No. 14, F sharp minor
Book 1, Prelude No. 15, G major
Book 1, Prelude No. 18, G sharp minor
Book 1, Prelude No. 19, A major
Book 1, Prelude No. 20, A minor
Book 1, Prelude No. 23, B major
Book 2, Prelude No. 2
Book 2, Prelude No. 4
Book 2, Prelude No. 6
Book 2, Prelude No. 8
Book 2, Prelude No. 9
Book 2, Prelude No. 10
Book 2, Prelude No. 19
Book 2, Prelude No. 20

For a wonderful modern invention (the texture is more dynamic than strictly two or three part), see Shostakovich's Op. 87, No. 17, Prelude in A flat major.

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