Audio: primary triad (0:08)
primary triad plays a progression of primary triads on a bright Yamaha grand piano in the key of C. The primary triad figure shows the score.
A primary triad is a triad whose primary function is to promote tonality.
There are three primary triads: I, IV and V:
Tonic is the most important note in functional harmony. It represents home base in a key. The tonic triad is a primary triad and its function is to promote tonality. Functional harmony invariably starts on the tonic triad, returns to it at regular intervals, and ends with a tonic triad. The regular and repeated use of a tonic triad in a chord progression is the most important way to promote tonality in functional harmony. The tonic triad is notated I in a major key, and i in a minor key.
Dominant is the second most important note after the tonic. The dominant is a fifth above the tonic (or a fourth below) and the fifth is the most important interval after the octave. Notes that are a fifth apart are closely bound together, as demonstrated by the circle of fifths. The dominant triad, V, is the primary triad with the fifth degree of the scale as its root. It contains the leading note of the scale, which has a strong tendency to move by step up to the tonic. This is why the primary function of the dominant triad V is to precede a tonic chord in a V-I or V-i progression. A dominant triad leads a chord progression home to the tonic triad. Many, if not most, functional chord progressions are more or less elaborate ways to get from the tonic to the dominant and back again to the tonic. The dominant triad is a V chord in both major and minor keys.
Subdominant is the third most important note. The subdominant is a fifth below the tonic (or a fourth above). The subdominant triad is the primary triad with the fourth degree of the scale as its root. It has an ambiguous character and is classed by some as a primary triad and by others as a secondary triad. It functions as a primary triad in the chord progressions IV-I and iv-i. At other times it functions as a secondary triad in support of the primary triads. The subdominant triad is a IV chord in a major key and a iv chord in a minor key.
Chord function is a term applied exclusively to the triad. Extending a triad to make a larger chord with four or more notes changes the quality of the chord but, importantly, does not change its function. It does not matter how a chord is extended or altered in shape, its function remains that of the root note of the underlying triad.
Millions of pieces of music have been written using primary triads. Chord progressions using I, IV and V chords in a major key, and i, iv and V chords in a minor key, are widespread in tonal harmony.