Diatonic Theory - Basic Chord in a Key

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I'm sorry to say, but unless you have a killer ear and sensibility right out of the box, you need to take 
the time to understand this chord to scale relationship. It will be the best thing you can do in 
understanding what is probably the MOST confusing bit of theory a guitarist learns...Modes.

It's only confusing because most guitarists learn the patterns without the theory...so, here's the theory.

Below are the basic Major and Minor triads\chords build from a Major Scale and it's Modes, and the Mode Names.

I'll show them from the C Major Scale...the Key of C...chords build from the C Major Scale...or the Chords of the Key of C...

The notes of the C Major Scale, or the Key of C are: C D E F G A B C

Triads/Basic Chords:

C
Dm
Em
F
G
Am
Bmb5
C

You can see we used each note of the scale as a Root. Then starting with each note as a Root, we used every other note (the 1, 3 and 5) to come up with these basic triads/chords, like so:

C Ionian = CDEFGABC = 1,3,5 from this scale is CEG = C Major
D Dorian = DEFGABCD = 1,3,5 from this scale is DFA = D Minor
E Phrygian = EFGABCDE = 1,3,5 from this scale is EGB = E Minor
F Lydian = FGABCDEF = 1,3,5 from this scale is FAC = F Major
G Mixolydian = GABCDEFG = 1,3,5 from this scale is GBD = G Major
A Aeolian = ABCDEFGA = 1,3,5 from this scale is ACE = A Minor
B Locrian = BCDEFGAB = 1,3,5 from this scale is BDF = B Minor b5
C Ionian = CDEFGABC = 1,3,5 from this scale is CEG = C Major

Commonly each of the scale steps or chords are give a Roman Numeral and are commonly listed like this:

I = C
ii = Dm
iii = Em
IV = F
V = G
vi = Am
vii = Bmb5
I = C

The Roman Numerals are commonly used to show "scale steps", as it relates to a Major Scale: C = I, D = II, E = III, F = IV, G = V, A = VI, B = VII, C = I again.

You can see from the basic Triads that some Modes create Major chords, and some Modes create Minor chords.

The upper and lower case Roman Numeral or "scale step" tells you whether a chord built from the step or 
degree of the Major Scale will end up being a Major or Minor chord.

Upper case = Major, and lower case = Minor.

More on the Roman Numerals in a bit.

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