How the Greek Modes Relate to Each Other
If you've been around musicians long enough, you may have heard about the Greek modes (aka Church modes) with the following names:
- Ionian
- Dorian
- Phrygian
- Lydian
- Mixolydian
- Aeolian
- Locrian
What is so special about them and what do they have in common? These scales are some of the most frequently used in Western music, and each one of them can be derived from others by starting at a new tonic. They are all just one scale played from seven different starting points. Every one of them is built from the exact same pattern of seven notes, the only thing that changes is the note you treat as home (aka the tonic).
One pattern, seven tonics
Play only the white keys on a piano, starting and ending on C, and you get C Ionian. Start that same run of white keys on D instead and stop an octave later, and you get D Dorian - Every Greek mode works this way: rotate which note you start on within one fixed collection, and you move between all 7 modes without ever picking up or dropping a single note. C Ionian's six other rotations are D Dorian, E Phrygian, F Lydian, G Mixolydian, A Aeolian, and B Locrian - all seven sharing one key signature, distinguished only by which note is the tonic. “A minor” being called the relative minor of C major is just this same idea under its more familiar name: Aeolian is the Greek name for the natural minor scale, and A Aeolian is C Ionian's sixth rotation.
This is what makes the modes “relative” to each other rather than “parallel” - for that distinction in general, and how modes compare when the tonic stays fixed instead, see Relative vs. Parallel Modes.
Because relative modes share every note, they also share every chord you can build from those notes. The triads available in C Ionian - C, D minor, E minor, F, G, A minor, and B diminished - are the same seven triads available in A Aeolian, D Dorian, or any of the other five. Nothing about the chord palette changes when you move between relative modes; only which chord acts as home does. C major resolves strongly to C because C is the tonic in that context - the same C major chord, played in A Aeolian's context, is just the bright major chord sitting on the third degree, with no special claim on being “home.”