To build a little on what Nathan's posted, let's look at a couple of scales.
<font face="monospace" size="3"><pre>
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15
C D E F G A B C D E F G A B C
D E F# G A B C# D E F# G A B C# D
E F# G# A B C# D# E F#G# A B C# D# E
F G A Bb C D E F G A Bb C D E F
G A B C D E F# G A B C D E F# G
A B C# D E F# G# A B C# D E F# G# A
</pre></font>
These are a few of the major scales in two octaves. By looking at them you can see where the "key signatures" in written music come from- C has no flats or sharps, while G has one, D has two, and so on. This is also why F is written as a "flat" key, because to notate the Bb note as an A# would make it impossible to write both A natural and A# on the music staff, so it's written (and referred to) as A natural and Bb.
As Nathan discussed, chords in our musical system are built in "thirds" or every other note from the scales. (called "thirds, because the notes are three scale steps apart,
including the note you start from). A major chord uses the 1st, 3rd, and 5th of the scale, so C major would be C-E-G, D major would be D-F#-A, E major would be E-G#-B, and so on. Adding the next "third" from the scale, or the 7th degree (scale note), gives major 7th chords or C-E-G-B, D-F#-A-C#, E-G#-B-D# etc. Adding the next "third" interval (the ninth) gives major ninth chords C-E-G-B-D, D-F#-A-C#-E, E-G#-B-D-F#. (You can see why it's called a "ninth"- we've just continued on upward from the seventh, skipping over the root we've already used in the chord). This contiunues on, always using stacks of "third" intervals, to build major 11th and thirteenth chords. (All the notes don't have to be played, just the ones that bring out the color of the chord.)
Those are how the chords in the Major family are built. The Minor family chords are all the same stacks of thirds, but always with a flatted third and seventh- so a C minor chord is spelled C-Eb-G, a C minor seventh is C-Eb-G-Bb, and a C minor ninth is C-Eb-G-Bb-D.
The third Chord family are the Dominants, which are formed the same way as the Major family, but always with the 7th note flatted. So- C7= C-E-G-Bb C9= C-E-G-Bb-D, etc.
Where this gets interesting is in the structure of "Key" itself- if you continue this chord building, always in thirds, starting from
each note in the scale, you get a series of chords. For example, from the C scale, four note chords from each note of the scale would yield:
C-E-G-B
D-F-A-C
E-G-B-D
F-A-C-E
G-B-D-F
A-C-E-G
B-D-F-A
Comparing these chords to their "own" scales, ie: comparing the D chord tones to the D scale, we find that these chords are:
C maj7, Dmin7, Emin7, Fmaj7, Gdom7, Amin7, Bmin7flat5. If we take the time to go through all the scales this way, we see the same pattern always holds true, and that for every scale we get a set of 7 chords that end up being 1 maj7, 2min7, 3min7, 4maj7, 5dom7, 6min7, and 7min7b5. This is really what we're talking about when we refer to the "key" of a song, and is why a song like "Help me make it through the night" is played as C-F-G in "C", D-G-A in "D", E-A-B in "E" and so on. Also why that simple three chord progression is referred to as a "I-IV-V", since those are the chords built on the first, fourth, and fifth scale degrees. Adding another chord such as in "Take me home country roads" would give us C-Am-G-F in the key of C, or chords built on the I-VIm-V-IV degrees. So transposing to any other key, we find that chord progression on the 1st, 6th(minor), 5th, and 4th degrees, or in G= G-Em-D-C. And so on. Although there are places where taste and musical "rules" may affect our choices, any of these chords can be extended to 7ths, 9ths, 11ths, 13ths... so a jazzier version of "take me home" might be played as: Gmaj7-Em9-D13-Cmaj11 for instance. This just scratches the surface of basic music theory, but it all starts and is built on those 7 numbers and the intervals they represent. How to find/use this on your guitar? Start with your open E9th tuning, compare it to the E scale above. If you're playing an E chord on say, strings 8,5 and 6, you'd be playing the notes E,G# and B. And that's why it's an E major chord. If you drop your thumb down to the 9th string, you're replacing the E root note with a D- the flat 7 note of E, and since the Bass or other instrument is probably "speaking" the root, you are voicing an E7th chord. Going back to your E major voicing, if you reach out and play the 2nd string D# note, you're adding the major 7 note into the mix for Emaj7. Play it and you'll hear the "Misty" schmaltzy or smooth jazzy sound that note adds. Flatting that string with your knee lever to D again gives you the flat 7 or Dominant sound, but this time on top. If you go to each chord position and run through all of the scale notes available on pedals and levers, and by moving the bar, you can see and find how to play any chord extention you want or need. Hope this helps some. Theory can be a deep subject, but just takes some study and the rewards are enormous.
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