In organ music, modal system is especially favored by the modern French composers (Alain, Durufle, Tournemire, Langlais and others). Messiaen had his own modal system of 7 modes of limited transposition. Today I'm going to focus on 7 diatonic (non chromatic) modes only.
There are 3 ways to learn diatonic modes.
1) By playing 7 white notes starting from each of the white keys:
From C - Ionian (CDEFGAB)
From D - Dorian (DEFGABC)
From E - Frygian (EFGABCD)
From F - Lydian (FGABCDE)
From G - Mixolydian (GABCDEF)
From A - Aeolian (ABCDEFG)
From B - Locrian (BCDEFGA)
2) By raising or lowering appropriate degrees of the natural major or minor scale:
Ionian (same as natural major)
Aeolian (same as natural minor)
Locrian (II- and V-)
NOTE: Lydian mode is the most joyful mode of them and Locrian - the most sorrowful. Locrian is very unstable because the distance from its 1st and 5th scale degrees is diminished 5th instead of perfect 5th as in all the other modes.
3) By adding one tetrachord on top of the other:
Ionian (Major from C and Major from G)
Dorian (Minor from C and Minor from G)
Frygian (Frygian from C and Frygian from G)
Lydian (Lydian from C and Major from G)
Mixolydian (Major from C and Minor from G)
Aeolian (Minor from C and Frygian from G)
Locrian (Frygian C and Lydian from G flat)
Try to play the notes of these 7 modes on your organ. When this becomes easy, practice building theese modes from the same note (Ionian from C, Dorian from C, Frygian from C, Lydian from C etc). This way you can master them from any of the keys you want. To make things easier, start with the white keys first and progress to black keys later.
Once these modes become easy for you, choose a simple meter and play melodies of predetermined length (like 8 measures) in any of these modes. This is a very important exercise in learning modal improvisation.
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