(con't from Part II)
The various modes have characteristics that create certain "colors" or moods which may help organ scholars better achieve their goals in composing or improvising.
IONIAN has no characteristic scale degree and is none other than the major scale by another name; it's exactly the same; it's the strongest mode of the 7 to our ears, and its attractive aspect is the tension and release that comes out of the half step between the 6th and 7th scale degrees; this tension is released as the 7th resolves back to the root; this mode produces an uplifting, innocent, happy, and upbeat mood.
DORIAN with it's characteristic natural scale degree 6 feels like a minor scale due to the minor triad up front, but here the 6th scale degree is natural instead of flat while the 7th is flat; this give it 2 curious characteristics: it sound melancholic but brighter and more positive than the typical minor scale, AND the 7th doesn't quite resolve which creates a sense of restlessness.
PHRYGIAN has a characteristic flat 2 scale degree and creates an ambiguous sound that leaves the listener uncertain of what they're hearing; because the 2nd is flat, it sounds strange to most people who are used to a whole step to the 2nd degree as in typical major and minor scales; its diminished dominant chord only adds to this strangeness, which can create a sense of mystery, dread, tension, and an impending negative event while still having a sense of warmth; it's also known as the Spanish Gypsy Scale.
LYDIAN has a characteristic sharp 4 scale degree and varies from Ionian by only 1 single note; while it's first chord is still a major triad the intervals are unexpected and surprising; they vary by 1 note, the sharp 4th, which strongly wants to resolve to the 5th, which should be used to advantage or it would be just as well to write in a major scale; this difference can be exploited very well to keep the listener engaged.
MIXOLYDIAN has a characteristic flat 7 scale degree and also varies from Ionian by 1 simple note; this is a popular mode for solo improvisations when in a major key because it provides a slightly unfamiliar counterpoint to help keep things fresh; it can provide a smoother, less innocent sound to otherwise happy music; it provides the same sense of not resolving like Dorian does if exploited.
AEOLIAN has a characteristic flat 6 scale degree and is familiar enough as the natural minor scale; it provides the sound of sadness, regret, resentment, or despair; it gives a slight sense of the Renaissance era at times due to the 6th and 7th scale degrees being flattened instead of natural; it will easily start sounding like the relative major if we use too many of its chords however, especially if we use its III (mediant) chord too much; this is because Ionian is far and away the strongest mode of the 7; give it a chance, and it will take over.
This is why the minor key uses a major V (dominant) chord and a raised 7th scale degree (leading tone) -- to draw the ear toward the minor tonic and away from the III chord as the relative major tonic (photo).
LOCRIAN has a characteristic flat 5 scale degree which makes it stand out, giving this mode its very dark sound; because so much Western music depends upon the major I and major V chords, we don't hear Locrian very much, if at all, due to its diminished tonic chord; because of this, many composers have gone so far as to categorize this mode as theoretical with no practical application at all; it provides a sense of brooding anger and sadness together, a sound that's much darker and dissident than any of the other modes provide.
With modes, we just have to underline the key chord more than normal; this "key chord" typically consists of the root and 1 (or 2) other note(s) which are characteristic of the mode and differentiate one mode from another.
With Lydian, for example, it's difficult to use more than just a couple of chords before it starts sounding like the relative major key, with just an odd focus on the IV (subdominant) chord.
The same applies to Phrygian and, to a slightly lesser extent, to Dorian and Aeolian.
If we check any tonal composition we find more than one key in it, accidentals, borrowed chords, altered chords, etc.; the same is true with modal music; some composers, such as Debussy, used a lot of modality in their works, but it's doubtful if there are some which are pure modal.
Therefore, in harmonizing a modal melody in pure modal style, we would try not using chords because there is a tendency to use them according to their functionality in tonal harmony.
Instead, we build our harmony as a result of counterpoint; we learn counterpoint in 3-4 voices (not always easy, it's true, but worthy) and use this kind of harmony, not the tonal one.
We would avoid tonic-dominant relations and, more importantly, we'd want to study the music of the old contrapuntal masters like Palestrina, Byrd, Tallis, and Frescobaldi, etc. (Bach is good too, but he's already tonal).
In modal harmony then, chords DO NOT have a function, so, in a sense, all chords are equal; a chord does not need to resolve to any other chord, but there is still a tonal center.
But because there is no "functional harmony" the chords DO NOT feel like they need to resolve to the tonic; each chord just floats there by itself as a stand alone entity.
In order to achieve this we have to avoid playing the diatonic tritone (all dominant chords have a tritone interval between their 3rd and 7th, which is known as the "diatonic tritone"), which is a very unstable and dissonant interval that wants to resolve; it creates a dissonance which sounds like a dominant chord and feels like it wants to resolve to the tonic chord, thus turning the music tonal.
This diatonic tritone is the basis of all tonal music, so, it's a delicate balance; we have to make the 1st degree of the mode sound like the tonal center, but we can't do it by using the function of the diatonic chords.
One way that composers get around this problem is by building chords with stacked 4ths instead of stacked 3rds, i.e. they use "quartal chords."
By building chords in 4ths they break the tonal anticipation of the dominant chord wanting to move to the tonic and they create a more ambiguous, vague, and modal sound.
Because modal chords don't have "functions," they don't have to go anywhere, i.e., they don't have to resolve to the tonic; they just float around; so therefore, modal songs usually don't have chord progressions; they just state the mode the song is in, and it's the performer's job to play any diatonic chords in the key of that mode and make their own chord progression.
The chord movements are made sparse and simple, not too busy, not too many chords, nice and boring; the chords are there just to create a harmonic underlay; quartal chords are often used to avoid tonal sound.
In a nutshell then, the difference in tonal and modal harmony is that tonality uses major/minor keys, functional harmony, and has a tonal center; modality uses all modes, no functional harmony, and still maintains a tonal center.
There are no absolutes when it comes to theory, especially since modes aren't frequently used these days all on their own; we're more apt to see a Dorian run, for example, in a piece in some minor key, as we find in the coda of the score for the d minor Op. 22 Postlude, or perhaps a short passage written in Aeolian (natural minor) in a piece in some major key, as we find in the 2nd half of the 11th variation of the score for the C Major Op. 4 Variations on a cantus firmus [See menu bar, Free Stuff].
The bottom line is that millions of musicians do just fine never touching the 7 modes; the organ scholar can get away with it too, but if we want to open up an entire extra avenue to help propel our own organ playing into uniqueness, if we want to have the capability of inserting a modal passage into something we're improvising or composing, and if we want to truly understand how early music was composed and why the ancient modal melodies in traditional chant, the Bach chorales, and certain modern compositions have the melodic curves they do, then we should take the time to learn how the modes are built and how to construct them on whatever scale we're using.