Audio: secondary dominant (0:08)
secondary dominant plays a chain of secondary dominants on an electric piano accompanied by a slap bass. The secondary dominant figure shows the score. The key is C melodic minor. All the chords are constructed from this key except the tonic i7, Cm7, which is borrowed from the parallel key of C natural minor.
A primary dominant is the note a perfect fifth above the tonic. A secondary dominant is the note a fifth above any degree of the scale other than the tonic.
Every key has a note called the dominant. This note is the primary dominant. It is the fifth degree of the scale. Every degree of the scale, other than the tonic, has a secondary dominant, which is a fifth above it.
The secondary dominant is nothing new. Nor is it unique to jazz harmony, it is also used in functional and pop harmony. It is yet another example of the application of the circle of fifths in music. A secondary dominant progresses by fifth to the note adjacent to it in the circle of fifths.
The purpose of a secondary dominant is to develop a harmony. It is primarily used to modulate from one key to another. It can also be used to convert a single chord into a chord progression.
The secondary dominant is mainly used for modulation. A chord built on a secondary dominant is labelled V/X where X is the degree of the scale. The primary dominant is V/I, or simply, V. The secondary dominant of V is II because II is the V of V, written V/V. In the key of C, for example, the primary dominant is G, and the secondary dominant of G is D. This secondary dominant can be used to modulate from the key of I to the key of V. The chord progression, vi-ii-V, becomes vi-II-V, Am-D-G, and modulation has taken place. The modulation can be short term tonicisation or it can be a longer term key change. This is a common use of the secondary dominant in jazz modulation, functional modulation and pop modulation. The secondary dominant is always a major chord when it is used for the purpose of modulation
Another use of the secondary dominant is to develop a chord into a chord progression. A chord built on a secondary dominant is labelled V/X or v/X where X is the degree of the scale. A single chord can easily be developed into a progression containing a chain of secondary dominants. The tonic I chord, for example, can be developed into the three chord progression, v/V-V/I-I, which simplifies to the familiar ii-V-I. Add another secondary dominant at the beginning of the chain, v/ii, to make vi-ii-V-I, a nice progression down through the circle of fifths. Bars 2-4 of secondary dominant contain a chain of seven secondary dominants.
The retrograde version of ii-V-I is I-V-ii. This is another progression through secondary dominants but this time by fifth up rather than fifth down. Progression by fifth up is less common than fifth down. Many reasons have been advanced for this phenomenon but no one really knows why fifth down sounds stronger than fifth up. Whatever, a progression by fifth up from I is easy to write. An example of progression by fifth up, IV-i-V, F7-Cm7-G7, is shown in the first bar of secondary dominant.
The leading note in a scale is a problematic secondary dominant. The chord on the leading note is a diminished chord and two notes have to be changed to make it a major triad or a seventh chord. Root progression by fifth down, viio-iii, is OK although the leading note does not, and cannot resolve, to the tonic. More problematically, a movement by fifth down from IV to viio moves by tritone and not by perfect fifth. Consequently, the secondary leading note chord, as it is called, resolves up to the tonic I chord in functional harmony, and does not move up or down by fifth. Jazz harmony sometimes follows the same route.