I'm firstly going assume that youve read some of the other articles on the website concerning chords and general music theory. If you haven't I suggest you have a look as there's some good articles floating around on here. If I repeat some parts of someone elses article, apologies.
Lets have a look at the basic chords in the key of C, we have:
1 2 3 4 5 6 7
Cmaj7 Dm7 Em7 Fmaj7 G7 Am7 Bm7b5
Now well use roman numerals to label each chord, this will make talking about each chords function much easier:
[tab]1 2 3 4 5 6 7
Cmaj7 Dm7 Em7 Fmaj7 G7 Am7 Bm7b5
Imaj7 iim7 iiim7 IVmaj7 V7 vim7 vii7b5[/tab]
Notice that the major chords have upper case numerals and the minor chords have lower case.
Chord Families
There are three chord families when talking about major key diatonic chords. Each family has a different harmonic function.
Tonic: I, iii, vi
Subdominant: ii, IV
Dominant: V, vii
and minor keys..
Tonic: i, III
Subdominant: ii, iv, VI
Dominant: v (or V), VII
The VI chord in both major and minor keys can be seen as either a tonic or subdominant depending on the chord progression. For example if you were playing a V7 followed by a vim7, that vi chord is going to sound like a chord from the tonic family. If you play a vim7 - V7 - Imaj7 progression though, the vi is going to feel more like a subdominant.
These 3 chord families and the movement between them is the basis of the majority of all western music. Usually you'll start at a place of stability (tonic), move away from it (subdominant), hit a place with tension (dominant) and then move back, or resolve, to a nice stable area. This tension and resolution is what makes chord sequences flow.
Cadences
FYI: Cadence comes from the Latin 'cadere' which means 'to fall'
A cadence is simply the movement from one chord to another in a piece of music, easy right?
A V - I is called an authentic cadence, or perfect cadence depending on who you ask. This is considered to be the strongest and therefore most important cadence. It gives a strong sense of resolution.
The next most important is the IV - I, or plagal cadence. This also gives a sense of resolution but not as much as an authentic cadence.
A deceptive cadence is the movement from V to any other chord other than I, so for example, V - vi. This is a weak cadence and lacks resolution, the movement to a V chord without following with the I can be used to create a sense of tension before eventually resolving to the I.
There are a few other cadences but we wont worry about them for now.
The root movements up of a 4th or down by a 5th are the strongest possible and are the building blocks of a great deal of jazz and classical music, not to mention pretty much every other genre. A good example of this movement would be a ii - V - I progression.
So if the V - I is the strongest cadence, what happens when I just keep playing V - I over and over? Glad you asked!
If you just keep repeating these strong movements, i.e. just keep going up by a 4th or down by a 5th youll end up with something called the cycle of 5ths. The whole cycle goes:
5ths--> <--4ths
C-G-D-A-E-B-F#/Gb-Db-Ab-Eb-F-C
Since this is basically a perpetual V - I cadence, try playing through the cycle of 5ths using dominant 7 chords, i.e C7-G7-D7 ect
Youll find that the chords will never feel like they get 'home' or resolve to any particular note or chord. This cycling by 5ths is used in many songs as a means to modulate into new key signatures.
If there's enough interest I'll cover the topic of modulation in another article.
That should be enough for a basic taster of chords and their diatonic functions. Hopefully this will she'd a bit of light into the dark evil place that is music theory! If anyone has any requests and wants to learn about any particular aspect of music theory, just let me know and I'll see if I can whip up another article on it for you. Mark Jones
/ my profile 
