The default triad is major, so you write C for the C major triad, and use an explicit m for the minor triad only. There is a certain logic to it nevertheless, which is why you don't really have to think about this. What is worse, the letter M doesn't apply to the triad, but to the 7th degree. A 7 by itself is minor, rather than major. The opposite default applies to the 7th degree. A minor chord with a minor 6th would be written "Cmm6": a Cm plus a m6. That is to say, if nothing else is written other than the digit 6, it's a major 6th. The confusing thing, perhaps, is that the "default flavor" for a 6th is major, but for a 7th it is minor. If there is just an "m", it makes the triad minor. In this case, the Cm6 means the "Cm" triad, plus a 6th. It turns out that things are the way they are because compactness of common forms beats consistency. Or, fortunately, depending on the point of view. Unfortunately, it is not consistent, either. The notation and terminology is such that "minor chord" doesn't simply mean "everything that has a major/minor designation must be minor". The Cm6 chord has a major 6th interval in it simply because the "minor" refers only to the second note it informs us that the second note is a minor third. So this same chord, C-Eb-G-A could easily serve as the subdominant in G minor, and in this case it would be derived from the G natural minor scale. Note that this chord quality is often used for the subdominant in minor keys. So if this chord sounds like a tonic chord in a piece, a more appropriate scale origin would be the: In practice, this added note is generally used for the harmonic interest it adds, not any melodic directionality. In any event, try not to get too caught up in the name, because as this wikipedia article explains, there are several different names for this chord.Ĭoncerning the scale the chord is built on, I would argue that it's not the melodic minor scale because that would imply that the sixth would have to rise through the seventh and up to the tonic. If you like you can think of it as a Cmin add6 chord. The word "minor" in this case has nothing to do with the quality of the sixth. The chord is called "minor" because of the quality of the triad C-Eb-G. An Ab would not produce this same tension.
The chord C-Eb-G-A has a major sixth in it because it provides the harmonic tension characteristic of this chord's sound, namely by virtue of the augmented fourth between Eb and A.