This is the greatest and funniest guitar lessons website I benifited from during all my career. What puzzles me is: 1 - how do I know when a song is in a certain mode? Is it only because that song start on a certain chord? 2 - what's modal tonality? 3 - How I can internalize these concepts so that they become natural in my playing?
Firstly, it's important to understand that not all progressions are modal. In time, you'll learn which chord relationships suggest modal tonality and which do not.
Think of modal key as the same as any other key - it's not about which chord you start on, although most songs do begin on their tonic "home" chord.
You'll know what the key of a progression is (modal or otherwise) because there'll be a feeling of resolution on a certain chord. It's the relationship between this tonic chord and the other chords that determines which mode it's in.
For example, the below progression clearly resolves to Em - Em becomes the tonic chord and the key center...
G / A / Em / Em / G / A / Em
Now, when learning modes of the major scale, you'll know that the minor modes are Dorian, Phrygian an Aeolian, so you'll know that as the progression is in E minor, it will be compatible with one of those minor modes.
Dorian. We know this because of the relationship between Em and the other chords based on the diatonic chord scale...
I w ii w iii h IV w V w vi w vii h I
w = whole step h = half step
The example progression used G and A major. Two major chords one whole step apart. This nearly always implies the IV and V (4 and 5) positions in the major scale. What are the 4th and 5th modes? Lydian and Mixolydian.
Therefore we can work out E minor's mode based on its relative position to these other modal positions.
Phrygian would lie a half step down from Lydian (which we've established is the G major (IV) chord in this progression), so Phrygian (iii) would lie on the note F#. Therefore Em must be Dorian, since that is the 2nd (ii) mode.
Let's look at another example.
A / A / G / Bm / A / A / G / Bm / A
A major steals the tonic on that one because we can resolve comfortably to it.
Again, to find the mode (if there is one), look at the relationship between each chord. It looks like we have another IV whole step V relationship between G major and the tonic A major. This tells us that G major will correspond with the 4th mode Lydian and A major with the 5th mode Mixolydian.
Therefore we can try A Mixolydian and see if it fits (which it does!). Incidentally, Bm is the vi (6) chord in this relationship, which always lies a whole step up from the V chord.
So simply by learning this chord scale...
I w ii w iii h IV w V w vi w vii h I
...you can identify the tonic chord's position and therefore it's related mode, because the mode number corresponds with the chord number.
I = 1st mode = Ionian ii = 2nd mode = Dorian iii = 3rd mode = Phrygian
By establishing a modal tonic, you automatically start applying modal tonality, because the relationship of that tonic chord with the other chords reinforces its tonality as the modal key/center of the progression.
When reading about this stuff it can seem overwhelmingly complicated. It's one of those things that is easier just to see in your own mind, to just "get it" through your own experimentation, than read an explanation of how it works.
First, make sure you've watched my two videos on understanding modes...
Once you have these interval relationships cemented VISUALLY in your mind and on the fretboard, spend time training your ear to the sound of these chord intervals.
For example, what does the major IV - V whole step movement sound like? Can you hear it being used in songs you listen to? What about I - vi?
If I played Am / Bm what relationship does this suggest? Answer: ii / iii because two minor chords a whole step apart always suggests ii / iii.
Once you can identify these chord relationships, you'll know which mode they correspond with.
Let me know any questions you have using the link below.