lets look at one, by its self and we'll analyze why this is only, a 2-5.
I'll use T Blocks voicings for this.
In the key of G major, the scale goes G A B C D E F# G
Here we have a A/GCE
you even talked about the concept in the "What chord is this" thread that to find a chord you spell it in thirds, so this one should be spelled, A C E G in root position. This follows the " build a chord off every other note in the scale" rule. In G major, this chord is a 2.
-Now you called it a 6. Which it could be, because they are spelled exactly the same way. Because , in addition to 2's, 6's are minor in nature in a major key as well as 3's. So it could be
a 6 in C major - C D E F G A B C, or
a 3 in F major - F G A Bb C D E F.
-Here is why it cannot be a 5 in a major key. Giving it the name of 5, it has to fulfill the function of a 5, and include a leading tone to the tonic.
So here we have our good ole Aminor 7 chord, spelled ACEG, if we call it a 5, that means the tonic would have to be D. So in D major we have D E F# G A B C# D. The leading tone in D major is the C#, which is necessary in the 5 chord to get back to the tonic. So in the A chord we have, we have a C natural. So automatically, by default this option is eliminated as a possibility. It cannot be a 5 chord in D major without a C#. For you advanced cats who are thinking about an A7#9 ie. A/C#EGB# or A7#5#9 ie A/C#E#GB# and how it has a C natural in it; it wouldnt be a dom7 chord at all with out the C#. Soon as you take it out, it looses its function as a 5. Again, the 5-1 option is eliminated as a possibility.
So to recap, it can be a progression that starts with a 2, 3, or 6, but cannot be any progression that starts with a 5.
Now for the ending.
Since we have 3 possibilities, this progression is ultimately determined by the two chords together, and not individually. So we, in essence need this next chord to tell us what key we're in and give us the ultimate analysis. Remember we have to analyze this in light of the whole progression, so we're analyzing the first chord and its possibilities for analysis.
So lets see if its a 3.
-If its a 3, then it puts us in the key of F major - F G A Bb C D E F, and the second chord of D/F#CE, by itself would have to be analyzed as a D9. in root position it'd be D F# A C E. Now we dont have an A in this chord, but that would not change the function of the chord, so its not important to have it in. I could develop that idea, but lets move on. So in the key of F major - F G A Bb C D E F, we have to decide if we can spell a D9 by the presence of the notes in the major scale of F; and we see that we cant because of the lack of an F#. This possibility is out. Next
Lets see if its a 6.
That would put us in the key of C major - C D E F G A B C. So our Aminor fits, but how would we analyze the next chord of D9? Could it be a 2? Lets see. D9 - D F# C E. We see we cannot use this analysis, because there is no F# in the key of C. The possibility of this progression being a 6-2 in a major is eliminated because of the F#.
So in the end, it can only be a 2-5
A minor chord going to a dominant chord one fourth up or one fifth down is a 2-5. Knowing that its a 2-5 gives you the key.
U know cs, all that gibberish would be correct IF the key for the first 2 chords was G major. Those 2 chords are 6-2 in C major or 5-1 in D major. U ever heard of the secondary dominant principle? Well that's what is occuring on the last chord of the set. Using the example:
A / G-C-E *6 in C major
D / F#-C-E *2 in C major, secondary dominant chord to 5, then probably will go to 1
or
A / G-C-E *5 in D major
D / F#-C-C *1 in D major, secondary dominant chord to 4, then probably a 2-5-1 to end
U think in the short run cs, I'm thinking long term. Make sense?
There is no A natural in Eb major. So that F13 cannot be called a 2, because its a borrowed chord from another key.
Just like I said above, SECONDARY DOMINANT, or in theory terms V of V. So, this can be called a 2.
