When playing solo over a chord, a scale should be chosen. When a musician choose a certain scale, what is the rationale behind?
I can come up with a few points but only poorly.
such scale
is there any other better ideas?
When playing solo over a chord, a scale should be chosen.
I would not necessarily choose a scale.
I would determine what key the chord is functioning in and then improvise in that key.
Ex. Dm7. You basic "chord/scale" teaching tells you to play a D dorian scale. That teaching does not explain the chord usually does not function in isolation but functions within a key.
So, if Dm7 occurred in progression Dm7 G7 C6, then we have a very recognizable progression functioning in C major. Roman numerals for it would be C: ii7 V7 I. You would improvise in C major.
what is the rationale behind?
Two reasons:
The three chords, or some similar passage, can be regarded as a phrase. You don't needed to select three "chord scales" in isolation, when you can just recognize the the passage as a phrase in C major. It's thinking of one unified idea versus three disconnected ideas.
Generally, you wouldn't want to choose a scale for a single chord that might conflict with the overall phrase tonality. Ex, with the Dm7 chord, or to make the point more emphatically, let's make the chord a plain Dm triad. Especially with the plain minor triad, many types of minor-ish scales will be suggested by "chord scale" teaching. You might choose D harmonic minor. That would conflict with the overall C major phrase. While not necessarily a musical disaster, temporarily tonicizing the Dm chord can be perfectly fine, but depending on the details you could make a muddle of the tonality.
There is no "proper" guideline for setting a scale to play over a chord.
There are some standard practices in thinking about the scale-chord relationships in various styles of solo playing, so the approach will be genre dependent.
A particular scale does not even need to be chosen to select notes to play over a chord. Using a scale is one way of simplifying the player's note choice in a solo.
When playing over a chord there will be notes that are "consonant" and "dissonant" to the chord tones, to various degrees. The most consonant notes will be the chord tones themselves. Non-chord notes that are in the same diatonic pattern as the chord notes will be the less dissonant notes. The "chromatic" notes will tend towards more dissonance.
Chords can suggest a scale to use, again depending on style. For example, if the chord progression is based on a diatonic I IV V progression ( C, F, G, in the key of C major), then the major scale can be used over any of the key's chords.
If a chord is extended, a different scale may be implied by the chord tones. For example, if the piece has a Dominant 7 chord, a scale may be built out of the four chord tones, with the implication that the scale will contain the intervals of the chord tonic, a major 3rd, a perfect 5th, and a minor 7th. Mixolydian (Major flat7) scale is an easy fit and is often suggested in jazz primers.
More complex chord spellings set a list of your primary consonant tones, and scale choice becomes a matter of experience and desired style.
