A typical modal tune may have only two or three chords, and each may last 8 or even 16 measures. In one sense, modal playing is much easier than playing changes, since it does not require your brain to do as much fast computation to constantly change scales. In another sense, however, it is more challenging, since you cannot merely string together rehearsed ii-V licks, nor can you rely on clever scale use and chord substitution to cover up basic problems thinking melodically.
Some music is often considered modal even though it follows traditional chord progressions such as the blues. The concept of modality has as much to do with what is done with the harmony as with its rate of change. In bebop derived styles, a soloist may sustain interest by his choice of notes over the harmony, including dissonances, tensions, and releases. For example, bebop players often enjoyed ending phrases on the raised fourth over a dominant chord, just for the effect that one note had. When soloing over modal music, there is less emphasis on harmonic choices, and more on melodic development. The ballad “Blue In Green” from Miles Davis’ Kind Of Blue has as much harmonic motion as many other tunes, and the chords themselves are relatively complex chords such as Bbmaj7#11 or A7alt. Yet the solos on this track do not exploit the harmony; instead they focus on melodicism of individual phrases. Bebop improvisers may emphasize the chordal extensions in their solos, whereas modal improvisers tend to emphasize basic chord notes. Bebop players are often more inclined to fill up all spaces with notes to completely define the harmony, whereas modal players are more likely to use rhythmic space as a melodic structuring element. Both approaches are valid, but it is important to understand the differences between them.
The Miles Davis tune “So What” on the album Kind Of Blue is the classic example of a modal tune. It follows a basic AABA structure, where the A section consists of the D dorian mode, and the B section consists of the Eb dorian mode. This yields 16 consecutive bars of D dorian at the beginning of each chorus; 24 counting the last 8 of the previous chorus. You may find yourself running out of ideas quickly if you limit yourself to just the seven notes in the D dorian scale, but that is the challenge. You cannot rely on the consciously hip sound of an F# over a C7 chord; you must play melodically with the notes you are given.
You are not completely restricted to the notes of the scale, however. As with ii-V progressions, there are some devices that you can use in a modal setting to add tension. One of the most popular of such devices is called sideslipping. Over a D dorian background, try playing lines based on Db or Eb scales for a measure or two. This dissonance creates a tension, which you can release by returning to the original scale. You can also use chromatic passing tones. For instance, over a D dorian scale, you might try playing “G, G#, A”, where the G# is a passing tone.
You can also vary the scale used. For instance, instead of D dorian, try a D natural minor, or a D minor pentatonic, for a few measures. You can also alternate a tonic chord with the dominant seventh chord in that key. For example, the chord associated with D dorian is Dm7. If you treat that as a i chord, the V7 chord is A7. So you can use lines from any of the scales associated with A7, A7b9b5, A7alt, or other A dominant seventh chords, at points in your improvisation. This will create a kind of tension that you can resolve by returning to the original D dorian scale.
For the most part, however, you should try to stick to the modal philosophy when playing modal tunes, and concentrate on being as melodic as possible with the basic chord and scale tones. Pentatonic scales are an especially appropriate choice in modal playing, since they narrow your choices to only five notes instead of seven, and further force you to think about using space and playing melodically. A similar sound is achieved by playing lines built from the interval of a fourth. This is called quartal harmony. It is particularly effective in modal tunes with few chord changes, although these types of lines can be used in other situations as well.