Motion from/to an inner voice, Untergreifen. This describes a linear progression at a higher level that connects a note of the Urlinie with a note of an inner voice at a deeper level. An initial ascent is an example of motion from an inner voice at a deep level. In mm. 1-4 of the aria, the ascending fifth B$-F is a motion from the inner voice (B$) to the Kopfton of the Urlinie. Mm. 5-6 exhibit another motion from the inner voice, from C to E$, 4 of the Urlinie. Such motions serve to prolong the deeper levels.
The example below illustrates both types of motion as they serve to prolong the
4 of the upper voice in Variation 16. Motion to an inner voice occurs in mm. 14-19 of the fugue, where a third progression, F-E$-D, prolongs the underlying harmony, I, by moving from the upper voice, F, to the inner voice, D.