Originally Posted By: ZeroZero
For me the major seventh sticks out big time, its the leading tone begging for the octave a very strong pull, not at all like its sister the dominant 7th. Using s major 7th to replace dominant 7th or vica versa. This again is a question of classifying dissonance, most text point out that the dominant seventh has a triton built from third to 7th - fair point, but to my ears a major seventh seems more dissonant. Why is this so?

The tension and resolution of the 7th vs. maj7th come from different places.

The physical explanation of tension can be often be explained by looking at the overtone series. Tones and invervals at the lower end of the series tend to be heard as less dissonant, while those at the upper end (excluding octave doublings) tend to be more dissonance.

I say "tend to" because musical context has a large part in how we interpret tension.

The first 8 overtones for C are C, C, G, C, E, G, Bb, C.

From that perspective, the Bb should actually be a member of the C major scale. So it's not surprising that it's fairly consonant. The decision of which way to lean the 7th of the scale - to the B or Bb has been a matter of debate over time.

Additionally, the Bb in the C7 chord signals that it doesn't belong to the key of C major. The most closely related major scale it does belong to is the key of F major, which is why the Bb helps the C7 nicely resolves to the F.

One can also propose a set of consonant intervals based on the overtone series: octave, fifth/fourth, third/sixth, seventh/second. From that perspective, the interval from the root to the flatted seventh ranks fairly high on the dissonance chart.

In contrast, the B in the Cmaj7 doesn't quite have the same physical justification. There are logical reasons why you might want to fill the gap between the 6th and 8th tones of the scale, but they aren't strongly supported by the physics of the overtone series.

The interval of a minor second also ranks higher than a major second as a tension.

Finally, the maj7 doesn't create the same sort of harmonic implications as the dominant 7th chord. Sure, it's a 7th tension (of sorts), but it doesn't create the same kind of V->I movement that dominant 7th creates.

So there is plenty of justification to saying the maj7 chord is more dissonant than the dominant 7th.

-- David Cuny

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