Prolongation, Auskomponierung. The extension (composing-out) of an underlying note, interval, or harmony (Stufe) by the introduction of additional notes at higher structural levels. The progressions of the middleground prolong the background, and the detail of the foreground serves to prolong the middleground. Prolongation may be contrasted in general terms with progression: prolongation represents the retention of a given sonority, whereas progression suggests a motion from one state to another. In general, progressions serve to prolong deeper levels.
The example, from Variation 18, shows a prolongation of the dominant harmony and of the
5 as the main upper voice note. The prolongation is carried out through linear progressions, a fourth, C-F in the upper voice, and a third, A-F in the lower part. The working out of this contrapuntal framework in a chromatic manner leads to the augmented-sixth chord at the end of m. 6.