Dahnielson

Harmony

Classic Harmony

Harmony define how different chords is related to each other, what function they have in a specific key. There are three basic functions, the tonic, subdominant and dominant (abbreviated T, S and D).

The tonic is a chord whose root is first note of the scale, and create the center of a key, the “home” if you will. While the subdominant, a chord with fourth note in scale as root, is “away from home” and the dominant, a chord with fifth note as root, is “returning to home”. Here illustrated in C major:

These three chord functions can also be substituted by other chords with related 3rd’s, like the parallel functions (abbreviated Tp, Sp, and Dp). Here in C major:

It’s also posible to substitute the tonic using the dominant parallel (Dp).

A summary of chords in C major and their functions:

I’ve added roman numerals to the chords above so you can quickly analyze a chord progression. Let’s use the II-V-I progression I previously mentioned as an example. And by looking at the staff above we can quickly determinate it’s a Sp-D-T progression (a jazzy variant of the S-D-T sequence.)

Jazz Harmony

Just as in classical harmony we have tonic, subdominant and dominant. And for the sake of discussing jazz harmony we can simplify by sorting the chords into general areas (here in C major):

Tonic Area

The tonic area includes chord I, chord III and sometimes chord VI. These chords are all similar because their roots are a diatonic third away from chord I. Because of that diatonic root movement of a 3rd is therefore seen as a weak progression. Chord III is often used as a substitute for chord I.

Subdominant Area

The subdominant area includes chord IV, chord II and sometimes chord VI. Chord VI is a diatonic 3rd away from both chord I and chord IV hence it can be seen either as tonic or subdominant. In jazz the II chord is more common than the IV chord as a subdominant, but it also functions very commonly as a lead in to the dominant V chord. While IV is very common as a subdominant in blues.

Dominant Area

The dominant area includes chord V and chord VII. The dominant quality of a chord is usually defined by flattened 5th interval, which creates what is thought of as a need to resolve to a chord that sounds more at rest (like a tonic chord). The VII is rarely used as a dominant.

Cycle of Fifths

The diatonic root movement by a third is weak as the second chord has three out of four notes the same as the previous one. However, the strongest root movement is downwards by a fifth (or upwards by a fourth). If we continue moving in fifths we have a progression which goes through all twelve notes available in western music and arrives back where it started:

        C     F    

    G             Bb(A#)

 D                   Eb(D#)

 A                   Ab(G#)

    E              Db(C#)

        B    F#(Gb)

Secondary Dominant

Secondary dominants are often used in jazz to create chromatic interest since they are not diatonic. And in practice any chord that is not a tonic chord can be preceded by a secondary dominant.

A secondary dominant is a fifth degree chord in the key of the current dominant chord. For example in the key of C, instead of preceding G7 by Dm7, the G7 is preceded by D7 - the dominant chord in the key of G. An actual modulation does not occur because the G is not a tonic, it is a G7 and therefore functioning in the key of C. In this case it is created by chromatically altering the 3rd of the Dm7 chord from minor to major.

Common Progressions

The traditional I-IV-V sequence is not that common in jazz, instead the I-II-V is favored. The II-V-I can also be substituted with a III-VI-II-V-I turn around, which is variation on I-VI-II-V one of the most basic chord progressions in jazz.