The opposite of augmentation, a transformation of the subject (theme or a motive) where the note values are smaller, the movement faster and the overall length of the subject entry shorter.
Extended counterpoint often transforms its primary material, a recurring subject or motive in a variety of ways. Transformations retain something of the original to maintain coherence, but, through modification, add diversity, perhaps even a new perspective on the nature of original material. For additional commonly used transformations, see inversion and augmentation. These devices are not unique to fugue, but occur frequently in other compositions using imitative counterpoint including invention, and classical forms such as string quartet and symphony.
Since diminution makes the melody line shorter and faster, it does not readily jump about as an obvious transformation but rather blends within the countrapuntal texture as mostly a compelling detail. Again, it is a fascinating game to seek it out where it occurs. Of the various transformations, it is the least frequently used (or detected!).
Here is an example of a subject and its diminished counterpart from Bach's Well-Tempered Clavier, Book 2, No. 9 in E major:
Fugues using diminution.
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