Update: This material was developed back in 1992, as part of a music theory compendium I used to hand out to my guitar students at the time. The entire compendium will be available in PDF-form soon.
Understanding Music Modes
The Diatonic Scale
The music modes are the seven scales (or modes) we can get out of the diatonic scale used in most western classical music. In our western music history, our notes are based on the chromatic system, where you have 12 half steps in an octave. The diatonic scales/modes, consist of a specific series of seven half and whole steps from the 12 chromatic notes.
These seven steps are: Two whole steps, a half step, three whole steps, and a half step.
Depending on which note we set as the start note, we will get a different combination of these seven steps. It is a common misconception to explain modes as variations on the major scale, when the major scale is just one mode out of the seven diatonic modes, then called the ionian mode.
The Seven Modes
So, starting a scale on each step of the diatonic scale, gives us a different mode (see fig. 1).
If we take the diatonic steps in the key of C, we get C-Ionian (which is the same as the C-major scale), D-Dorian, E-Phrygian, F-lydian, G-Mixolydian, A-Aeolian (which is the same as the A-minor scale), and finally B-Locrian (see fig. 2). All these modes share the same key, notes and chords - however, they sound completely different in their modal context.
In western music, we usually think in terms of major or minor keys. And most people understand the theory behind C-major and A-minor sharing the same notes and chords; as in A-minor is the parallel key to C-major. It is also clear to most, that C-major and A-minor doesn't sound the same. One is uplifting and happy, whereas the other sounds more mellow and sad. If we now think of these as two modes (ionian and aeolian), we understand that the other modes also have their distinct sound or feel to them.
Dorian might be considered melancholic, mysterious, thoughtful or reflective. Mixolydian has a more heroic feel to it, and is often used in music surrounding sports events. This is highly personal of course, and can vary based on the musical composition and arrangement elements too. The point is, that the seven modes gives us more tonal contexts to work with than only major and minor. In fact, we have three major modes and four minor modes (see fig. 3).
As we can see from the figure - Lydian, Ionian and Mixolydian have the major third interval in relation to the tonic. Whereas Dorian, Aeolian, Phrygian and Locrian all have the minor third interval in relation to the tonic.
Here are the different modes we get from the C-tonic, in relation to the corresponding key (see fig. 4).
"Lighter" And "Darker" Modes
It's easier to see what's going on if we sort the modes from "light" to "dark". For each mode we go down, one note is changed (see fig. 5).