Diminution
The diminution of note values.
What it means
Diminution (German Verkleinerung) designates the rendering of a theme in smaller note values, mostly halved, so that it sounds twice as fast. The sequence of notes and the intervals are preserved, only the durations are shortened. Its counterpart is augmentation, in which the note values are lengthened.
Musical use
In the fugue, diminution belongs among the contrapuntal arts with which a theme is transformed in the course of the piece. If the theme appears diminished simultaneously with its original shape or its augmentation, relationships arise in which the same line lies stacked at different speeds. Such combinations often stand toward the end of a fugue and form a climax of the contrapuntal work. In motivic-thematic work, diminution serves compression: a motive that recurs in smaller values produces urgency and acceleration.
In mensural notation the procedure had a meaning of its own. Through proportion indications, a voice could be instructed to perform its values reduced in a particular ratio; in this way a cantus firmus could be led at different speeds against the other voices.
The word diminution has a second use: in the performance practice of the sixteenth and seventeenth centuries it designated the ornamenting of a melody, in which long notes are resolved into faster figures. Both meanings have in common that smaller values arise, but they mean different things.
In the fugue it serves as a means of intensification, often combined with the normal shape and the augmentation in other voices. In the Renaissance, diminution also meant the ornamenting of a line through fast figures, an improvisatory technique for which there were dedicated method books. Both meanings are to be distinguished from context in a given case.
Performance practice
When a theme appears diminished, make it recognizable despite the faster movement — the head must stay distinct. With combinations of different speeds, take care that both shapes are audible. And mark diminished entries in the notated text; on the page they at first look like new material.