We can number the modes as follows:
- I - Ionian (or major scale)
- II - Dorian
- III - Phrygian
- IV - Lydian
- V - Mixolydian
- VI - Aeolian
- VII - Locrian
Now, let's say we're in the key of C, and we decide to switch the scale to Dorian (applying R-2-b3-4-5-6-b7-O), which would be,
C - D - b(E) - F - G - A - b(B) - C
or simplified,
C - D - Eb - F - G - A - Bb - C
Now we may ask, given these notes, what key am I in? Since we now have C Dorian (instead of Ionian), we count one in reverse, and that gives us Bb; yes, given the new resulting notes we're now in the key of Bb!
Let's try another example - here 's F Ionian,
F - G - A - Bb - C - D - E - F
Now, let's say we (arbitrarily) decide to go to F Locrian: R-b2-b3-4-b5-b6-b7-O, which gives,
F - b(G) - b(A) - Bb - b(C) - b(D) - b(E) - F
or simplified,
F - Gb - Ab - Bb - Cb - Db - Eb - F. What key am I in? Since Locrian is 6, we can count back (or in this case, forward by one) and that puts us in the key of Gb!
Yet another cool way of looking at this insanely crazy topic of modes!
Just to wrap this up with some terminology I've been reading about:
- Any collection of 7 notes placed in the intervals R-W-W-H-W-W-H (whole/half step) forms a diatonic collection (i.e. a major scale) given a chosen root note R
- From this scale, any note we choose to play from as a starting point puts us in a mode (the default is Ionian, if we start from R)
- Playing in Aeolian is the same as playing the natural minor scale of that root (e.g. if root is C, then playing from A is in Aeolian mode, and we are also playing the natural (A) minor scale of the C (major) scale