Guitar Chords - Advanced Information

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 bases all chords on the dominant scale.   This model is surprisingly emcompassing - 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 all based on the dominant scale, they are based on their own scales.   While this is technically correct, it leads to some ambuguity, and debates with 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:
    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, like most scales.

Now it gets tricky:

An Am6 is also based on the A minor scale, but which one?   There are lots of minor scales; some with a flattened 6th, others with a natural 6th.   Minor chords are based on the dorian minor scale, and for A dorian minor, this is:
    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.

Here are some other common minor scales:
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:
    A  B  C  D  E  F#  G#  A
which 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 dimished chords are the best 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 is:     G  A  Bb  C  Db  Eb  E  Gb  G
The 7th note in this scale is E (the note you'd normally hear as a 6th).   And unlike most scales, this one has 9 notes in an octave!

So looking at the 3 common diminished chords:

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

You can probably see what a mess this is with chord descriptions.   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 desciption commonly shown for a diminished chord including the 6th note.   At the end of the day, the right chord is the one the writer intended, nothing more.

It gets worse.   For Gm7-5, the F is not even a note in the common diminished scale shown above, but it is the 8th note in the other 'half-step whole-step' variation.  

Note that all 3 of these common diminshed 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 normally hear as a 7th)
And unlike most scales, this one has only 7 notes in an octave!
Of course we all play a G7+5 as:  G  B  D#  F.