The following article has a daunting amount of text, it IS theory after all. It might be best to just play through all the examples and listen. You will find you can take away some of the ideas even if you don't fully understand the explanations.
If you study Classical music at a conservatory you know all your fundamental theory, and will gradually build on that until you get to some pretty interesting and complex stuff. Applied Chords and Modulations, Neapolitan Chords and sequences are tricks that could be used to make your guitarring/songwriting much more interesting and sophisticated, if you have a spare three or four years to do a Bachelor degree (After passing a classical guitar audition of course). Otherwise read on and I will explain some useful ideas in succinct terms.
Standard chord progressions:
You might know that each key (Major or minor) has a dominant chord that leads strongly back to the tonic chord. You don't? Okay, observe:
Play this chord progression, in which G7 (the dominant) leads unequivocably back to C (our tonic). Try stopping after the G7 chord, feel the "tension". Now play the final C chord and feel the "release".
G7 -> C
dominant -> tonic
tension -> release
The chord I haven't described is the F chord in the second bar, which is called a pre-dominant (for obvious reasons, coming before the dominant and all). So the chord progression is actually:
|Tonic |Pre-Dominant |Dominant |Tonic|
Although this is a strong chord progression, it is common (ah, the old "one-four-five") and unoriginal, so let's see what we can do to change that!
There are loads of other ways to get from our C to our G7, which can be much more interesting than the normal F. These are different pre-dominants: ii6, The Neapolitan 6, the V/V, vii7/V etc. but instead of explaining the theory let's play them and hear the difference:
IV as pre-dominant (original progression):
ii6 as pre-dominant:
Neapolitan6 (N6) as pre-dominant:
V/V as dominant of dominant:
vii7/V as dominant of dominant:
Some of these pre-dominants change the sound radically while some are much more subtle, and of course it is personal taste which ones you use and which ones you ignore.
Instead of the dominant chord why not use a diminished chord for something a little darker?
Now, in my opinion, we are getting somewhere much more interesting than where we started. Try using the Bdim7 after each of the pre-dominant alternatives, and compare it to the original progression. Find something that you like and weave in an original melody to really make it your own.
Now we can start putting extra chords in each bar to make the movement more complex (ie more interesting still!). This is achieved by using the dominant of a chord that is not the tonic. This is called an 'applied dominant'.
Listen for the tension chord at the end of each bar leading to the resolution at the beginning of the next.
and for those who is still following the theory, here is a mixture of applied dominants and applied diminished chords:
|C C7|F F#dim7|G G7|C Bdim7|
Where the hell did you pull that from?:
So we started with a simple chord progression and altered and changed it until we arrived at the chord progression which is immediately above. Some of you might be wondering at this point how I worked out what each chord was, and how you can apply these things to different keys. Explaining this is a little harder but if you can get a grasp of some of the following you will find it very useful.
Each key has a set of chords built from the scale, the following being the chords of C Major:
C Dm Em F G7 Am Bdim
and every Major key has the same pattern of:
Major minor minor Major Dominant minor diminished
So substitute the scale notes with roman numerals and you get a framework for the chords of any given major key:
I IIm IIIm IV V7 VIm VIIdim
Notice the dominant there as V7, the pre-dominants IIm and IV, and of course our diminished chord VIIdim. Here is the pattern in some other Major keys:
G Am Bm C D7 Em F#dim
D Em F#m G A7 Bm C#dim
Now, when we're using an applied dominant, we are actually borrowing the dominant from another key, like how we had D7 going to G in our chord progression. This is called a 'tonicisation' of G (temporarily making G the tonic). So to work out which dominant chord you can use as an applied chord, you first need to think of the chord you are resolving to as the tonic, then work out the dominant of that key.
Here the dominant resolutions you can use as applied dominants (V7 -> I):
B7 -> E
E7 -> A
A7 -> D
D7 -> G
G7 -> C
C7 -> F
F7 -> Bb
Bb7 -> Eb
Eb7 -> Ab
Ab7 -> Db
Db7 -> Gb
C#7 -> F#
F#7 -> B
Or if it helps to think of it in a different way, build your dominant chord from the fifth of the tonic (or 'temporary tonic'). For a diminished resolution, build the diminished chord from the note a semitone below the tonic:
F#dim7 -> G
Bdim7 -> C
G#dim7 -> A
C#dim7 -> D
Edim7 -> F
It is relatively simple to apply this theory to minor keys, as dominants resolve to minor chords in the same way as they do to Major:
B7 -> Em
E7 -> Am
A7 -> Dm
As for a template for the chords of a minor key, the following works best:
Im IIdim III IVm V7 VI VIIdim
Am Bdim C Dm E7 F G#dim7
Some these chords did not occur naturally in the key, but were 'borrowed' from A Major, as we really like to have E7 as our dominant (rather than G7) and G#dim7 (rather than Bdim7).
Here are some of the chord progressions used in this article, changed to sound minor:
|Cm C7|Fm D7|Gm G7|Cm G7|
So mess around with that and I'll listen to what you come up with when it's on heavy rotation on MTV.
Here's something I came up with:
Here's the roman numerals:
|Im|Vm|IVm|Applied VII|V7|Im|N6 VIIdim|Im|
Some Chord Shapes:
Dm/F C#/F F#dim7 Bdim7