This descending third progression, as a middleground replica of the Urlinie (or fundamental descent) presents a number of opportunities for what Schenker calls 'form generation':
- the note marked with a star could be prolonged into a substantial section in the dominant.
- could be prolonged so that the initial descending third progression spanned a section in the tonic (as in Three Blind Mice); could then be prolonged into a section in the dominant.
The rest of this subsection of SchenkerGUIDE show various other progressions that can generate form in this way. In the first level middleground, most of the rules of Species Counterpoint must be strictly adhered to - there can be no parallel fifths, dissonances must be properly treated and there can be no large leaps etc.
For progressions closer to the foreground of a piece of music the rules are gradually relaxed. Schenker suggests that the rules of Species Counterpoint are essentially about coherence - counterpoint that breaks these rules does not make sense.
In a real piece of music, progressions in the first level middleground have to be so strict because they support large sections of the music. Diminutions closer to the surface of the music can rely on the coherence of the middleground progressions they prolong to help them make sense.
Schenker was very particular about which progressions can appear in the first level middleground for the same reasons - only some prolongations of the fundamental structure are robust enough to support large spans of music.