fretski wrote: ↑Mon Nov 30, 2020 3:36 pm
An M7 is the addition of the 7th note to the major triad.
Right, although good to note that it's the addition of a
major 7th to the major triad.
Major triad + major seventh = Major 7th chord (e.g, Cmaj7, CM7, CΔ7)
Major triad + minor seventh = Dominant 7th chord (e.g., C7)
Minor triad + minor seventh = Minor 7th chord (e.g., Cm7, C-7)
Minor triad + major seventh = Minor major 7th chord (e.g., CmMaj7, CmM7)
Both involve the additon of one other note to the major triad, so why the placarding as if they were on different plants? Why not just CM7 and CM9?
One reason is because often a chord is
not just adding one note to the triad, and there needs to be some way to concisely indicate exactly what notes are being included. And there's the not only the maj/min thirds to deal with, but also maj/min sevenths (as well as augmented/diminished intervals).
In chord construction, you have your triads. When you stack a seventh, you get a seventh chord. When you stack more notes on top of a seventh, you get an extended chord. This is kind of inherent in the way chords are constructed. So if you say "extended chord" to musicians, they'll assume you mean "some sort of seventh chord with extension beyond the seventh." Note that seventh chords, themselves, are not usually considered "extended chords". An extended chord includes and extends on top of the seventh.
So instead of just thinking about "triad plus some notes" you need to think about those three categories,
each of which builds on the former: basic triads, seventh chords, and extended chords.
This means that the chords like C9, C11, C13, etc. all mean "a dominant 7th chord with something added": they imply the inclusion of the dominant 7th (the triad + a minor seventh). They don't just add one note to a triad, they add two or more.
And there's also major 9th chords. Those are the major triad, the major seventh, and the 9th. So C9 would be C E G B♭ D, but Cmaj9 would be C E G B D.
In contrast, Cadd9 means "a C major triad with only the 9th added -- don't include the 7th." An add9 chord steps outside the "normal" pattern of chord construction in this way.
So to address your question about why not just "CM7" and "CM9", it's because extended chords include a seventh, and also because we need a way to distinguish between the major and minor seventh intervals, as well as the major/minor third in the triad. And because it's possible to add a 9th (for example),
without including a seventh. The somewhat convoluted naming convention of chords allows you to specify all of this once you know its "rules."
Under those rules, "CM7" means "a C major triad with the major seventh". "CM9" means "a C major triad with the major 7th and the 9th". If you want to specify "C major triad with an added 9th" you use "Cadd9" to indicate that it's only adding the 9th and not building on the seventh chord. If you want a dominant 7th chord with an added ninth, that's "C9". And so on.
Sixth chords (e.g., C6) are not considered "extended chords" because they do not build on top of a seventh chord (they don't "extend beyond the seventh"). They're really "add chords" like the "add9".