The I chord: the tonic
Just as the tonic note is the most stable note of the key, the major chord built on the tonic, called the I chord, is the most stable chord within a key. It is composed of scale degrees 1, 3, and 5. As in the hero’s journey myth, the hero departs from home, embarks on adventure, and ultimately returns; most effective music begins at the tonic, departs to other harmonies, and ultimately returns “home”. Reduced to the most basic harmonic level, most songs would be best represented by the I chord. However, while the tonic sounds stable, satisfying, and at rest; music only becomes meaningful when it explores other harmonies.
The V chord: the dominant
The diatonic chord built on scale degree 5, called the V chord, is a major chord consisting of scale degrees 5, 7, and 2. It possesses a strong inclination to resolve to the I chord due to the presence of scale degrees 7 and 2. The V chord often facilitates a return to the tonic, creating a sense of finality due to its tension with and resolution to the tonic, and is the most commonly used dominant chord.
The IV chord: the subdominant
The major diatonic chord built on scale degree 4, called the IV chord, consists of scale degrees 4, 6, and 1. This is the most common chord of predominant function, and it naturally resolves to the V chord due to the proximity of scale degrees 4, 6, and 1 to 5, 7, and 2. The subdominant has a more stable relationship with the tonic than the V chord. Its root note, scale degree 4, is one of the most consonant intervals with the tonic, and the 5th of the triad is the tonic note. As such, it facilitates harmonic movement without a strong tendency to resolve to the tonic.
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