All seven modes of the major scale can be defined by the location of one crucial interval - the tritone.
A tritone is an interval made up of 3 whole steps (6 half steps), thus the name "tritone". This interval is exactly one half of an octave. This is clearly seen on guitar by playing an open string, then the 6th fret of the same string, then the 12th fret which we all know is one octave above the open string.
Now let's move on and apply this idea to the major scale modes. The major scale is made up of the following series of whole (W) and half (H) steps...W W H W W W H.
Now let's apply a number to represent each scale degree...1 W 2 W 3 H 4 W 5 W 6 W 7 H 1. This may look confusing, but it's pretty simple. From scale degree 1 we move up a whole step (W) to arrive a scale degree 2. From there we move up another whole step to arrive at scale degree 3, etc.
Can you find the tritone?
Remember, a tritone is made up of 3 whole steps. In the major scale, the tritone is found between scale degrees 4 and 7. Observed harmonically, the "Ionian" sound can be boiled down to Maj7(add11). No other major scale mode gives us that harmony.
Let's apply this same principle to the second mode of the major scale (the Dorian mode).
Our series of whole and half steps shifts by one scale degree resulting in this...1 W 2 H 3 W 4 W 5 W 6 H 7 W 1. To clarify, in this example scale degree 1 was (and is) scale degree 2 of the 1st major scale mode.
Now let's find the tritone. In this mode, the tritone is found between scale degrees 3 and 6. The location of this interval in this mode results harmonically in a Minor 6 sound. No other mode of the major scale contains both a minor 3rd and a major 6th.
Here is a snapshot of this principle at work in all seven modes of the major scale.
If you're just learning about modes this may be somewhat confusing, but for those of you who have been struggling to understand the modes I hope this sheds some new light on the subject for you.
I'd love to hear your comments, questions or feedback of any kind! Thanks for reading!
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