And another fantastic post from C.;;;; Just perfect.
Modulation can really break you out of what I refer to as "Box Thinking" or just pertaining to the diatonic theory within a certain Key.
Take for example the Key of Em; If you play a progression using Em-Am-G-Bm
i-iv-III-v,,,, it remains diatonic with Em. What if you wanted to change the Bm (v) to B7 (V7) for a different effect to the progression; i-iv-III-V7......
The i-iv-III are diatonic but the V7 isn't (B7= B-D#-F#-A). How could it work?
The 3rd (D#) isn't in the Key of Em, but all the other notes in B7 are.
It would work if you played by starting with the Em scale and maybe improv into the Em pentatonic over the diatonic chords and substitute with the E Harmonic minor scale over the B7 chord, which would tie in the non-diatonic part of the progression, leading back to the i chord.
This would be considered using Parallel minor scales in the progression.
Em scale (Natural minor); E-F#-G-A-B-C-D (Em pent;= E-G-A-B-D)
E Harmonic minor scale; E-F#-G-A-B-C-D#
