Secondary Dominant Chords
Secondary dominant chords occur in every genre of music and it's important to understand their function in harmony. Most songs would be pretty bland if they consisted only of diatonic chords.
Now remember that the primary dominant,
the V7 chord,
which resolves down a fifth to the I chord, is an important building block of music. Well, the other diatonic chords each have their own "secondary" dominant chord.
Here are the secondary dominants in C major:
The II-7 chord (D-7) has the secondary dominant of A7. This is called the V7 of II or V7/II and occurs in that classic progression:
Cmaj7 A7 D-7 G7.
The secondary dominant of the III-7 (E-7) is a B7 and is labeled V7/III.
The secondary dominant of IVmaj7 (Fmaj7) is a C7 and is labeled V7/IV.
Yes, even the primary dominant (G7) has it's own secondary dominant which is D7. The V7/V occurs extremely often, especially in classical music.
The secondary dominant of VI-7 (A-7) is an E7 and labeled V7/VI.
However, there is rarely a V7/VII chord so don't worry about that one.
Secondary dominants will often be used in the middle of a progression or at the end to transition to a new section. Secondary dominants are also used to modulate to a new key by becoming the new primary dominant of the new key.
Also, secondary dominants, as well as primary dominants, don't always have to resolve down a fifth. Often dominant chords will "deceptively" resolve to a different chord. Deceptive resolutions sound great because they keep the energy of the progression building.
Here's an example of secondary dominants and deceptive resolutions in the begining chord progression of the standard, On the Sunny Side of the Street:
Cmaj7 (Imaj7), E7 (V7/VI ), which then deceptively resolves to Fmaj7 (IVmaj7), G7 (V7), which also deceptively resolves to A-7 (VI-7), D7 (V7/V), D-7 (II-7), G7 (V7), Cmaj7 (Imaj7).
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