Schenker uses the word diminution in its ancient sense of
replacing a given long note with two or more shorter notes, thereby
enlivening the texture and adding to the motivic content. Schenker
also uses the term diminution to refer to the surface of the music,
sometimes to the foreground level.
This definition contrasts with the more common use of the
term in reference to the repetition of a theme in shorter notes, hence
at a faster pace.
The example, from the foreground of Variation 15, illustrates
how the rhythmic activity found in the diminutions of this variation provides
a multitude of brief statements of the main motivic ideas, the rising third
and the upper neighbor.