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The purpose of the chord pages on my site is to make it easy for anyone (beginner to advanced) to find and understand the notes used in chords. To do this I have described my own model that builds chords from note options within the Mixolydian (dominant major) scale. This model is surprisingly encompassing - it covers nearly every chord in common use! You don't need to know any of the following to work out how to play published chords, in fact I recommend not even reading it until you're comfortable using my basic model to work out and understand chords!
Chords are not really based on the dominant scale, they are generally based on scale of the music your playing and the relationship of the current chord to that scale. While this is technically correct, it leads to some ambiguity, and pointless debates from purists vs common understanding.
Here are some non-ambiguous examples:
An Am7 is based on the A minor scale:
A G7 is based on the G dominant scale when playing in the key of C major.
Now it gets tricky:
An Am6 is also based on the A minor scale, but which one?
There are many minor scales; some with a flattened 6th, others with a natural 6th.
If we are in the key of G, then Am6 would be based on the A Dorian scale:
But if we are in the key of C major, then technically, the 6th note of the Am chord is F and not F#. However, you still play the Dorian minor 6th (the F#) in Am6 unless it's specified that the flattened 6th should be used.
Here are some other common minor scales that show different 6th and 7th notes:
Now it gets ambiguous:
The overlapping descriptions of diminished chords are a good example.
The diminished scale is commonly played as alternate whole-step then half-step, or vice versa depending on the musical context.
It doesn't matter for this explanation, so here's whole-step, half-step scale:
The 7th note in this scale is E (the note you hear as a major 6th interval). And unlike most scales, this one has 9 notes over an octave! Of course, we don't label note names accordingly - the 7th is the dominant scale 7th, not the diminished scale "8th"!
So looking at the 3 common diminished chords:
Popular sheet music shows dim (or O) to mean the Root, m3, -5, 6 chord. Some people will say this is technically not correct, but like it or not, this is the description commonly shown for a diminished chord including the 6th note.
Note that all 3 of these common diminished chords are covered in my simple dominant scale model, anyway. It is the application of different scales, and overlapping use of names that causes confusion here. I recommend that you use non-ambiguous descriptions when publishing your own chords, such as:
Augmented scales have the opposite problem.
Here is the G augmented scale, made up of all whole-steps:
You can probably see what a mess this is with chord descriptions. At the end of the day, the right chord is the one the writer intended, nothing more.