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DIATONIC HARMONY
Diatonic Harmony is fundamental to understanding Western music.
Diatonic = 'involving only notes that belong to the key, without chromatic alteration'.
Harmony = 'the combination of simultaneously sounded musical notes to produce chords and chord progressions'.
That sounds pretty complicated, but it just means working out the chords that belong to the scale.
TRIADS (THREE NOTE CHORDS)
A triad is a three note chord. We can build a triad upon each note (or scale degree) of a major scale. If we only use notes from the scale, the triads will be diatonic.
Triads in a major key will always follow this pattern:
Major Minor Minor, Major Major Minor, Diminished
In classical analysis of chords (harmony), we use Roman numerals to identify the chords. Upper case Roman numerals are major, lower case are minor.
1 = I 2 = ii 3 = iii 4 = IV 5 = V 6 = vi 7 = vii
The major triads are built upon notes 1, 4, and 5 of the major scale. We call these primary chords.
Chords 2, 3, and 6 are minor chords - we call them secondary chords.
For the sake of simplicity, we will ignore chord 7 (the diminished chord) for now. It's a little strange, and is nowhere near as common as the major and minor chords.
PRIMARY CHORDS
As stated, the chords built on the 1st, 4th, and 5th degrees are all Major chords. We call these chords Primary Chords. Each Primary Chord has a name, to help us to identify the role (or function) it plays in the key.
I = Tonic (this chord functions as 'home')
IV = Subdominant (this chord creates 'movement' - it takes you somewhere)
V = Dominant (this chord is as far away from 'home' as you can get - there is a strong sense of needing to go back 'home').
CADENCE
Movement away from the tonic chord creates tension, and moving back to the tonic creates resolution.
The strongest resolution is created by moving from chord V to chord I (Dominant to Tonic). This is called a Perfect Cadence.
The second strongest resolution is achieved by moving from chord IV to chord I (Subdominant to Tonic) We call this a Plagal Cadence.
SEVENTH CHORDS (FOUR NOTE CHORDS)
A seventh chord is a four note chord. We can build a seventh chord upon each note (or scale degree) of a major scale. If we only use notes from the scale, the seventh chords will be diatonic.
Seventh chords in a major key will always follow this pattern:
Major 7 Minor 7 Minor 7, Major 7 Dominant 7 Minor 7, Minor 7 Flat 5
Again, we will continue to ignore chord number 7. However, observe that chord V is labelled as Dominant Seventh chord.
DOMINANT 7th CHORDS
There is only one Dominant Chord diatonic to a Major key - it is always chord V.
The formula for a Dominant 7th chord is 1-b3-5-b7.
The b7 is the characteristic pitch that differentiates the Dominant 7th from Major 7th Chords.
BLUES HARMONY
Blues music goes against the conventions of traditional harmony. The three Primary Chords are still the foundation of Blues chord progressions, but all of the chords are Dominant Chords.
Notated below is an example of a 12 Bar Blues progression in the key of C. Notice that all of the chords are Dominant 7th chords.
BLUES MELODY
I have written out the notes of C7 chord, followed by the notes of a C Blues scale.
The b3, b5, and b7 are considered to be the 'Blues Notes'.
You can see that the 1, 5, and b7 are shared by the chord and the scale. However, the scale contains a b3, while the chords contains the natural 3.
Typically the b3 will be bent a little sharp, so that it sits somewhere between the b3 and the 3. This is called a 1/4 step bend. This is one of the most characteristic sounds in Blues music. Instruments that cannot produce 1/4 step bends will often precede the 3 with the b3, as illustrated by the example below.
THE BLUES CONNECTION
Personally, I believe that the the last 4 bars of a 12 Bar Blues progression have gone on to inspire countless Classic Rock songs. I think that rock musicians were so accustomed to hearing these 'changes' from playing blues songs, that they naturally began to use these chords when they wrote their own songs.
I have written out the last four bars of a 12 Bar Blues in 5 common rock keys.
There are many songs that use these chords, and there are many variations of the order that they are played in. However, the thing that makes it sound more 'rock' is to treat the V Chord as the tonic, or main chord.
You can see examples of the progressions here.
The harmonic ambiguity (part major, part minor) of the V Chord means that, if you are careful, you can juggle between major and minor when you improvise over it!
Technically, by making the V chord the tonic, we have shifted keys into the Mixolydian Mode.
V would now be I
IV is now bVII
I is now IV
However, for the sake of simplicity, I will continue to refer to the chords as V, IV, and I. I think it make it easier to identify that the tonic has 'dominant' qualities, which suggests that if you are careful, you can use both major and minor pentatonic, and blues scale to improvise over it.
The following diagrams detail the Primary Chords as moveable Barre Chords, so you can play these Rock Chord Progressions in any key.
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