Harmonic Progressions – T-PD-D Functions

In the lesson on Harmonic Progressions–Down a 5th, we saw the entire down a 5th cycle:

which can be seen in the following progression:

However, the viio chord, a diminished chord built on the note a half-step from tonic, usually functions as the leading tone triad and so it usually goes to I (not iii).

If the viio goes to I and the IV still goes to viio (down a 5th progression), then the cycle changes a bit to:

The bottom part of this progression, I IV viio I, can be seen in the progression below:

From this progression, we can see the groupings of the chords into functions appear.

Tonic–Pre-Dominant–Dominant Functional Movement

In the diagram below, with viio going to I, the chords start to divide themselves into three groups or functions:

Tonic: vi and I
Pre-dominant: ii(7) and IV
Dominant: V(7) and viio

Tonic chords are those chords that are at rest – I and vi (which can be used in place of I). Pre-dominant chords are ones that come before dominant (ii, IV). Dominant chords are that lead back to tonic – the V and the leading tone triad, the viio. Both of the dominant chords (the V7 and viio) contain a the tritone (4-7 scale degrees) leads to 3-1, which are part of the tonic chords.

The following diagram can help us think about these chords in function categories:

Let’s look at some simple progressions that contain these chords:

The above progression shows the down a 5th progression (ii V I) as PD-D-T.

The above example shows the IV-viio6-I as PD-D-T. Remember that the viio chord usually appears in first inversion – viio6 – to minimize the tritone; if it is in first inversion, then the tritone is not between the bass and another voice. The inversion does not change the function; the viio6 is still pre-dominant in function.

Now let’s look at a couple of more complicated progressions:

In the above progression, both VI chords function as tonic. The first VI is an extension of I and the second is a replacement for I at the cadence (forming a deceptive cadence).

The final progression (see below)

The III chord is a tricky chord. Since it shares 2 notes with I, it can be used as tonic.

But it also shares 2 notes with V. Especially when it is found in first inversion at a cadence (iii6-I) it can be thought of as a dominant substitute – a Vsub6 chord.

Below is an example showing both uses of the iii chord.

In the above example, the first iii chord is an extension of I (forming an arpeggiation in the bass of the tonic triad). As such, it is functioning as tonic.

The second iii, the E-/G is marked as a V chord since it is really functioning as a dominant chord; the E in this chord (making it a iii) is really substituting for the D and appears as a pedal tone.

Complete Function Diagram

The complete function diagram, including the iii chord in both of its functions (and with chords that usually have 7ths), is shown below: