Note: C D E F G A B
Chord: E F G A B C D
G A B C D E F
C is the Tonic or I chord
D is the SuperTonic or II chord
E is the Mediant or III Chord
F is the SubDominant or IV chord
G is the Dominant or V chord
A is the SubMediant or VI chord
B is the Subtonic or VII chord
The basic flow of a chord progression is
Tonic - SubDominant - Dominant - Tonic
in C: C | F G7 | C
note in these chords that all notes are diatonic to C
(all chord tones occur naturally in C scale)
F A C
G B D F
C E G
Let us look at a chord progression then:
C Amin F G
Everyone's heard this one. It's classic fifties, but it illustrates the point.
C Tonic (start), Amin (substitute subdominant), F (subdominant), G (dominant)
The dominant leads you right back into the tonic.
Now here's where we can have some fun. Chord Substitutions.
Look at the notes of a G9 chord:
G B D F A
Look at a Bmin7(b5) chord:
B D F A
The B chord is just the G chord without the G.
It follows that in place of the G9 (essentially a G7, or a G for that matter) you can use a Bdim or Bmin7(b5).
Intervals of Thirds always work like this.
The Tonic of C can be replace with Emin, which is a good trick.
The Dmin can be an F and vice versa.
Emin to G
F to Amin
G to B
A to C (not common)
B to D
NOw let's see why the Dominant to Tonic works:
G B D F
C E G C
B goes to C. D goes to E. F goes to G.
The C chord is just revoiced to be
G C E G.
Now there are other tricks you can do with this. Note that the most used dominant chord (the V) is a perfect fourth below the Tonic (I). Because of this, you can use patterns of acending 4ths.
F Bb Eb Ab Db Gb B E A D G C F
There's a whole chord progression right there hidden behind the circle of 5ths. It's even the right number of chords to make a 4/4 song of.
This trick works in other progressions as well though.
Check out this variation of the fifties progression above:
C |Amin |F |D G |
The Dmaj isn't actually diatonic to the key of C, but it flows from the F well enough and check out what it does:
D G C. You have that ascending 4ths. D is the V of G. It's called the "Five of Five," and is a great trick to use.
Jazz Chords, and Tritone Substitution.