After the I, IV, and V chords, the
II chord is the next most likely to be used. It might be a major chord. It
might be a minor chord. It might be a 7th chord. But it is the most likely
chord to occur after I, IV, or V.
In the key of C, that means that some form of the D chord is
the 4th most likely chord to occur. Maybe D7, maybe Dm7,
maybe just D.
And when it does occur, you can then predict which chord
will come after it - the V chord. And after that, the I
chord. This progression is known as the II - V - I chord progression. And it
happens over and over again.
Each chord is a 4th higher than the previous chord. In the
Key of C, after the D chord is used, you go up a 4th (4 scale notes - in terms
of traditional harmony it is a "perfect 4th) to G. Then after the G chord
is used, you go up a 4th to C to complete the cycle.
The 'VI - II - V7 - I' Chord Progression
Chord II might occur as a major chord or as a minor chord or
as a 7th chord, but however it appears, it almost always follows the chord
progression of II, V, I.
If II is in its 7th form, such as II7 or IIm7, that even
strengthens its predictability more - those 7th chords really want to move up a
perfect 4th.
In the key of C, and you are on a Dm7 chord, and you have an
85% chance of predicting that the following chords will be G followed by C. You
don't know when, of course, but just knowing the likely path gives you an
advantage.
Chord VI doesn't show up as often as chord II, but it is
predictable. It is going to move up a 4th to a II. It
is pulled irresistibly toward II.
When a VI shows up, the next chord will be a II. Close to 90% of the time VI moves to II. And you
already know that II likes to move to V, and V likes to move to I.
You'll notice immediately that each progression is up a
perfect 4th (or down a perfect 5th same thing). This is a reference to
the Circle of Fifths.
Embedded Chord Substitutions
There are many variations to this
progression. Usually the two "middle chords - the VI and the II - are
played as minor chords, and are then known as vi and
ii (use small Roman numerals for minor chords). Usually, too, all the chords
except the I chord have a 7th in them - in other
words, in the key of C: C Am7 Dm7 G7.
You can use 1/2 step slides as connective
chords between the main chords. You might insert a Bb7 between C and Am7 as a
chord substitution. That would let you "slide in" to Am7 in a smooth
fashion. Then you might slide into the next chord - Dm7 - from 1/2 step above -
Ebm7. Or you might slide into G7 - Ab7 to G7. Or you might slide into C the
second time around: G7 Db7 C.
So
starting with C Am7 Dm7 G7, you
might end up with C Bb7 Am7 Eb7 Dm7 Gb7 G7 Db7 C.