Dominant Function

When a chord is said to have dominant function it means the chord creates a sense of tension, which can be smoothly resolved by changing to the chord a perfect fifth below it. Dominant function gives chord progressions a sense of movement.

Perfect cadence (V7-I)

When this resolution happens at the end of a phrase, with the V7 resolving to the I, it's called a perfect cadence. This is a common way to end a musical phrase or to establish a new key center.

Dominant seventh chords

As explained in Primary Chords, the V chord is called the dominant chord, because it's a perfect fifth above the tonic.

When we stack another third onto the V triad chord (adding a minor seventh interval), it becomes a dominant seventh chord.

This is the origin of the term dominant seventh chord: the dominant chord with its seventh interval. Here it is voiced as the E7-shape chord:

The dissonant tritone

The tension in a dominant seventh chord mostly comes from the tritone interval between the 3 and ♭7 of the chord. This is the most dissonant interval in Western music. Listeners don't want to rest on this sound; they tend to want it to resolve to a different sound. This is how the desire for movement is created.

The tritone interval is between the 3 and ♭7 in the chord. For the V chord, those notes are the 4 and 7 of the key. Press the strum button beneath the diagrams to hear the tritone interval.

Resolving tritone to major third

The reason the V7 chord resolves so nicely to the I chord is because the dissonant tritone in the V7 chord subtly shifts to become the stable major third in the I chord. The 4 and 7 that make up the V7 chord's tritone each move a half step in opposite directions, becoming the 3 and 1 of the I chord's major third.

Press the play button in the two diagrams below to see and hear this resolution play out from the perspective of both chords. Then press the strum button for each diagram to hear the full effect of the two chords resolving.

Secondary dominants

The V chord is the only dominant seventh chord that occurs naturally when harmonizing a major key. But the dominant function, the effect of resolving from a dominant seventh chord to the chord a perfect fifth below, is so useful for creating a sense of motion in a chord progression that composers often apply it to other chords as well, using chromatic notes from outside the key to make the dominant seventh chord.

When a chord other than the V chord is turned into a dominant seventh for this purpose, it's called a secondary dominant chord.

For example, the I chord can be played as a dominant seventh in order to resolve to the IV chord:

This works for minor chords, too. For example, the iii chord can be played as a III7 dominant seventh to resolve to the vi chord.

Diminished vii° chord

The other chord with dominant function in a major key is the vii° chord. The three-note version of this chord is a diminished triad, which is rarely used in popular music.

But notice that the V7 chord contains all the same notes as the vii° diminished triad:

The vii° triad is essentially a rootless V7 chord. They can often be used interchangeably in chord progressions.