Harmonic functions

There are 3 harmonic functions:
  1. Tonic is the most stable, balanced, relaxed and consonant function of a tonality. Is the most important function, but we can also say that it is the most "boring" and that in some way "calls for" a change.
    • Chords with tonic function in major tonality or mode: I, IIIm and VIm; C major example: C, Am and Em
    • Chords with tonic function in minor tonality or mode: Im and bIII; A minor example: Am and C[7Maj]

  2. Dominant is the most tense, unstable and discordant function of the tonality. After hearing a dominant chord, ear expect to settle in a tonic chord.
    • Chords with dominant function in major tonality or mode: V7 and VIIdim; C major example: G[7] and Bmº
    • Chords with dominant function in minor tonality or mode: V7 and VIIdim; A minor example: E[7] and G#[7dim]

  3. Subdominant function also sounds somewhat tense and unstable and expect to settle in tonic, although with less force then dominant. Sometimes subdominant unstability is described as "introspective," "intimate", while dominant unstability is more "exciting".
    • Chords with subdominant function in major tonality or mode: IV and IIm; C major example: F and Dm
    • Chords with subdominant function in minor tonality or mode: IVm and IIº, #V and bVII; A minor example: Dm, Bmº, F[7Maj], G[7]
Alterations indicated in brackets are [optional]. Dominant chord usually takes 7th.

With these notions, the problem of "find the right chord" is reduced to "find the appropriate function", and then choose a chord that roles this function.
In principle any of the listed meet, but the specifics of the melody could "preferable" to one and even "forbidden" to others chords.

Listen Rice pudding in C major, using only C as tonic chord, G as dominant and F as subdominant chord:


Listen it now harmonized with C, Am and Em as tonic chords, G and Bmº as dominant chords and Dm as subdominant chord:

Sounds pretty strange, but acceptable because harmonic functions are the same.

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See also:
Harmonic field