If not consonance, what actually makes a chord be stable?
Tonal context as defining consonance and dissonance
Consonance does make a chord stable, but stability and instability are relative. A chord isn't so much stable or unstable as it is more stable or less stable. So, considering chords that all share the same root, and played individually (rather than as part of a progression), a major triad is highly stable, a diminished triad is highly unstable, and
m6 is less stable that a major triad but more stable than
m7, which is more stable than the diminished triad.
However, context matters. If you have a piece predominantly in C major, and you suddenly toss in a D major triad, that triad will sound far less stable than if the piece had been in D major or G major. The same holds true for any chord. The more alien it is from the primary (local) key area, the more unstable its sound.
Interval content as defining consonance and dissonance
One also must be careful is defining the "consonance" of a chord. Above I've discussed consonance within a given harmonic context. However, the video, contrary to my statement above, claims that
m6 is more dissonant (and by my argument, therefore, less stable) than
m7. This is also true.
A variation on my statements above: within the context of a minor key, the
i6 chord (i.e.,
m6 built on the tonic pitch) is more stable than the
i7 chord (i.e.,
m7 built on the tonic pitch). In the context of the key,
i6 is more consonant than
i7. This is the context of the chords as discussed in the video, but the video presenter is using a different context to define the consonance and dissonance of the chords.
Consider the chords in isolation, and look at the intervals present in the chords (in root position; dissonant intervals are in bold).
||intervals from root
||... from 3rd
||... from 5th
m3 P5 M6
m6 has two dissonances —
M2 — while
m7 has only one —
m7. Further, the
A4 in the
m6 chord is the "most dissonant" interval in either chord. So both in terms of number of dissonances as well as the level of dissonance,
m6 is more dissonant than
Chord function as defining consonance and dissonance
Finally, in the view of "music theory", the
m6 chord is functionally in the same category as a minor triad — stable, though with an "extra" note that should be resolved. In jazz and pop, the need to resolve is removed, and the
m6 is treated as actually equivalent to the minor triad).
By contrast, seventh chords are viewed as functionally distinct from triads — less stable, with a greater need to resolve. While jazz and pop again allow for seventh chords as consonances, in the case of the video under discussion, the presenter is taking the more "classical" view of things.
What makes a chord "resolutive"?
Stability; therefore, consonance — but "consonance" within the larger harmonic context, as opposed to "consonance" in terms of the specific intervals within the chord.