Hello, the current topic is minor mode. Why can only II degree follow major IV in minor scale? Can't quite figure this one out.
Schönberg is treating the minor scale here with the options to raise the sixth or the seventh scale tone, forming a rising and a falling scale. A few pages before this Schönberg postulates four "turning point laws" for the minor scale, which try to keep the tonal character of the minor scale clear. These are:
- Erster Wendepunkt gis: gis muß nach a gehen, denn gis ist nur wegen des Leittonschrittes zu nehmen. Keinesfalls darf g oder f gebracht werden, ebensowenig fis (wenigstens vorläufig)
- Zweiter Wendepunkt fis: fis muß nach gis, denn es wurde nur wegen des gis gebracht. Keineswegs darf g oder gar f folgen. Aber auch (mindestens vorläufig) nicht e, d, a usw.
- Dritter Wendepunkt g: g muß nach f, denn es gehört der fallenden Skala an. Keinesfalls dürfen fis oder gar gis folgen.
- Vierter Wendepunkt f: f muß nach e, denn es gehört der fallenden Skala an. Keinesfalls darf fis folgen.
- First TP g#: g# has to go to a, for g# is only used because of the leading tone interval. In no case may g or f be used, nor f# (at least for now)
- Second TP f#: f# has to go to g#, because we only use it for the g#. By no means may g or even f follow. But also not (at least for now) e, d, a &c.
- Third TP g: g has to go to f, because it belongs to the falling scale. In no case may f# or even g# follow.
- Fourth TP f: f has to go to e, because it belongs to the falling scale. In no case may f# follow.
So keeping by these rules we see that the f# of the
IV# chord may only rise into a g# (or remain on f#).
Furthermore at this point Schönberg is only considering chord changes that share at least one note in common, which he calls harmonisches Band (harmonic bond) (so at this point Schönberg does not consider changes like
IV# - V# will actually cause a problem, as f# → g# would require parallel shift of the chord).
But thus the
IV# in question has only a limited amount of options:
- D → B minor (
IV - II)
- D → G major (
IV - VII)
- D → B dim (
- D → F# dim (
IV - VI#)
- D → F major (
IV - VI)
- D → A minor (
IV - I)
But if we consider the TP rules (f# may only be followed by g#) only the first options remains:
- f# → g is not allowed
- f# → g is not allowed
- f# → f# would be allowed, but as the last clipped sentence on your pictures says this diminished chord would need to be resolved by a 4th upwards (cf. the early section about the 7th scale degree in major), f# would need to go into b, which is not allowed, as f# → g#.
- f# → f is not allowed
- f# → e is not allowed
Thus only the given chord change is possible.