Harmony Theory deals with the chords within the particular key. Notes of C-major key are shown on Figure 1. Each scale degree is subscripted with the Roman numeral. The first, fourth and the fifth scale degrees have specific names: T(Tonic), S(Subdominant), D(Dominant).
See the main chord types on Figure 2. The first in the notation is the scale degree the chord is built from. In this example all chords are composed from the first scale degree C-major key so all of them begin with I. Also letter 'T' can be used instead. Digits after the scale degree number define the chord type:
There is a four-voice chord division in the Harmony Theory. They are:
Triad is a chord consisting of three notes. To obtain its four-voice form we must double one of the notes. Usually the basic chord note is doubled. On the Figure 3 you can see the four-voice triad form built on scale degree I in C-major key (number ). In this case note Do is doubled. You can arrange chord notes in the four-voice form by several ways. Notes are arranged descending. Any chord note can be placed in the first position (Soprano voice). Number on the Figure 4 shows the chord arrangement, where in Soprano voice the first chord sound is located (C). Number on the Figure 4 shows the chord arrangement, where in Soprano voice the second chord sound is located (E). The chord arrangement, where in the upper voice the third chord sound (G) is shown with the number on the Figure 4. The chord notes can be arranged one by one or next by one. The one by one arrangement is called 'tight' (see number on the Figure 3) and the next by one arrangement is called 'wide' (see number on the Figure 3). There’s also a 'mixed' arrangement. It’s a combination of two previous arrangements (see number on the Figure 3). Bass can hang behind the Tenor by two octaves. Thus there are always options of the Bass positions as you build up the chord. See Number on the Figure 4 for three optional Bass positions. Now you can see that there are many ways of the chord arrangement.
There are the rules that define how one chord notes transform to another chord notes. These rules are called the 'voice-leading' rules. The basic are:
There are three functional chord groups related to the scale degree the chord is build from. They are:
Functional group defines a chord role in the scale.
Triad T5/3 is a main member of the Tonic group chords. It implements function of stability. T5/3 is the target of any chord progression. Chords of the Subdominant group and the Dominant group sound unstable, but different. Chords of Dominant group have the VII, the most instable scale degree that requires a resolution to the tonic. Thus these chords sound strenuously and also have a strong tendency to resolve to the tonic. Chord D5/3 is the main member of Dominant group. Chords of Subdominant group do not contain the scale degree VII and sound less strenuously compared to the Dominant group chords. Chord S5/3 is the main member of Subdominant group.
Within the harmonic progression each following chord sounds more strenuously then the previous. So there is a main rule used for chord progression composition: the chords of Subdominant group cannot succeed the chords of Dominant group.
The triads built on scale degrees I, IV and V are the main triads. The triads built on scale degrees II, III, VI and VII are called subsidiary triads.
The triads built on scale degrees II and IV are the subsidiary triads of the Subdominant group. Within each group the chords sound with different degree of instability. The more joint sounds has the chord with the tonic triad, the softer it sounds. The triad built on scale degree VI has two joint notes with the tonic triad and thus sounds very soft. The triad built on II scale degree on the opposite has no joint sounds with the tonic triad and sounds more strenuously. (Reminder: in the chord progression each following chord is more strenuous than the previous one). The rule follows for the Subdominant chords positions re each other within the chord progression. It’s typical that the more strenuous triad built on scale degree II appears in the chord progression after the triads built on scale degrees IV or VI. The triad built on scale degree IV may appear in the chord progression after the weakest Subdominant, after the triad built on scale degree VI.
Subsidiary triads of the Dominant group are the triads built on scale degree III and VII. They also sound with different degree of instability. The tensity degree of the chords in the Dominant group has the following progression: the weakest is the triad built on scale degree III, then the main Dominant – the triad built on scale degree V, then the most strenuous – the triad built on scale degree VII. In the chord progression these chords may have the same order.
Triad inversion sounds dissonance and strenuous by its nature, therefore in the chord progression it usually follows the triad 5/3 type. For example, S6 follows S5/3, but never vice versa. The same with the seventh chords – the seventh between the Bass and the upper sound adds tensity to the chord sound. Thus in the chord progression usually the seventh chord built on scale degree N follows the triad build on the same scale degree. For example, D7 follows D5/3, but never vice versa.
T-S-D-T is a pattern to build the chord progression.
The chord progression may be complete, i.e. contain Tonic and the Subdominant and Dominant chords. Thus it fully follows the pattern T-S-D-T. For example:
The chord progression may be incomplete. For instance, it may contain only Tonic and the Subdominant chords. These chords sound soft as they don’t contain the Dominant chords. For example:
The chord progression may contain only Tonic and the Dominant chords. They sound sharp. For example:
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