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 B C D E F G A
Every second note up to the 7th note gives an Am7: A C E G
This scale has 8 notes in an octave, like most scales.

A G7 is based on the G dominant scale when playing in the key of C major.
     G A B C D E F G
Every second note up to the 7th note gives a G7: G B D F
This scale also has 8 notes in an octave.

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:
     A B C D E F^# G A
The root, 3rd, 5th and 6th notes give Am6: A C E F^#
This scale still has 8 notes in an octave.

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:
A natural minor:
     A B C D E F G A
A harmonic minor:
     A B C D E F G^# A
A melodic minor (also known as the jazz minor scale):
     A B C D E F^# G^# A
A melodic minor is traditionally played as a natural minor when descending:
     A G F E D C B A

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:
G diminished scale:      G A B♭ C D♭ E♭ E G♭ G

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:

• Gm-5 (also known as Gdim or G⁰) has: G B♭ D♭
• Gdim (also known as G⁰ or Gdim7 or G⁰7) has: G B♭ D♭ E
• Gm7-5 (also known as 'G half-diminished' or G^Ø7) has: G B♭ D♭ F

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:

• m-5 for the occasionally used R m3 -5 chord
• dim for the (well known but technically incorrect) R m3 -5 6 diminished chord
• m7-5 for the R m3 -5 7 half-diminished chord

Augmented scales have the opposite problem.

Here is the G augmented scale, made up of all whole-steps:
     G A B C^# D^# F G
This scale doesn't even have a 7th note - the 6th note in this scale is F (the note you'd hear as a dominant 7th). And unlike most scales, this one has only 7 notes over an octave! Of course we all play a G7+5 as: G B D^# F.

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.

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