In order to have a full command of harmony and its harmonic relationships, it is important to understand the function of each diatonic chord of a key. The function of each chord will determine its inherent tendency to remain in position (stability/resting) or to have the need to move to another chord (unresolved/active). This knowledge will help in melodic harmonization, reharmonization as well as creating interesting, meaningful chord progressions.
There are seven chords in a major key corresponding to the seven notes in the scale, i.e. in a C major scale, the notes are C, D, E, F, G, A, B, C, and numbered accordingly as 1, 2, 3, 4, 5, 6, 7, 8 or 1. In solfege, this would read as DO, RE, MI, FA, SO, LA, TI, DO.
The seven diatonic chords built from this scale are: Cmaj7, Dmi7, Emi7, Fmaj7, G7, Ami7, Bmi7b5, and numbered accordingly as Imaj7, IImi7, IIImi7, IVmaj7, V7, VImi7, VIImi7b5.
The Tonic (I) category has a stable or resting sound, containing the restive tones of DO-MI-SO. Because they function this way, they can be found at the beginning or end of musical phrases providing stability. Chord I which has a major quality is the main chord in this category, followed by its two minor counterparts, IIImi and VImi. Notice none of these contain the active-sounding 4th tone or FA of the scale.
So in the key of C major, the Tonic chords are Cmaj7, Emi7 and Ami7.
The Dominant (V) category is the exact opposite of the Tonic, i.e. it has an active or unresolved sound. Chords in this category has a need to move to other chords, therefore creating movement within a musical phrase. Chord V7 is the main chord in this category, followed by the VIImi7b5.
So in the key of C major, the Dominant chords are G7 and Bmi7b5.
These contain both the 4th (FA) and 7th (TI) of the scale, which creates an interval of a tritone (three whole tones). It is because of this tritone interval that makes this chord sound active and needs movement or resolution.
The Subdominant (IV) category has a sound and function in between the resting and active category chords as it contains the restive DO as well as active FA. Because they function this way, they are usually found directly moving to Tonic chords or preceding chord V. Chord IV which has a major quality is the main chord in this category, followed by its minor counterpart, IImi.
So in the key of C major, the Subdominant chords are Fma7 and Dmi7.
It is important to note that all seven diatonic chords have been represented by these three main categories. Although there are seven different chords, they function in only three different ways, i.e. stable/resting (Tonic), active (Dominant), or half-active (Subdominant).
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