Secondary Dominant | The Non-family Member Chord

If you've ever had relatives come and visit you for an extended period of time, their presence can start to wear thin. You can probably relate if your home is the chosen meeting place for the holidays. But that doesn't mean you love them any less, even if you would prefer to see less of them.

So is the same for the family of harmony that we play in music. If we could just have some variety from the same old chords. You can in fact play some fresh harmony in the form of the secondary dominant. It's not complicated and I'll show you how simple it is.

To begin with, you should understand that any given key of music has it's own harmony. It's referred to as diatonic. Diatonic simply means "in the key of". Each tone in the key has it's own corresponding chord.

For example the key of C Major...

...has 7 corresponding chords that are numbered. They're combinations of Major and minor triads with 1 of them being diminished. It's common to use upper and lower case roman numerals for Major and minor triads respectively.

I is Major

ii is minor

iii is minor

IV is Major

V is Major

vi is minor

vii is diminished

So to clarify, the I IV and V chords are Major. These are referred to as the primary chords because they're the most used chords in a Major key of music.

The ii, iii, and vi chords are minor and the vii chord is diminished.  They are referred to as the secondary chords in a key of music. They're not used as much as the primary chords.

The V chord which is the dominant chord, has a very specific characteristic. That is, it has an aural pull to the I chord.If you add the flatted 7th tone (which is F) this creates a G dominant 7th chord. This has an even stronger pull to the I chord. There is only one naturally occurring dominant 7th chord in a key of music. It's referred to as the V7 chord in a Major key.

However, you can utilize a dominant 7th chord based on any other tone in the key.

Let me introduce to you secondary dominants. A secondary dominant chord is a dominant 7th chord built on a tone other than the V tone for the purpose of resolving to a chord other than the tonic.  In other words, it will give the quality that the V chord has to another tone.

Among the most common secondary dominant chords is the V7 of the V chord. If you're in the key of C Major, then the V7 chord would be G7.But the V chord of the V7 chord would be D Major. G is assuming the role as a I chord. This would make D7 the V7 chord.

Here's a typical vi-ii-V7-I chord progression

But with the secondary dominant chord, the ii chord which is D minor, is transformed into D7. It's still a ii chord but more specifically, it's a II7 (it's a Major chord instead of minor)

This is actually a significant chord substitution because there's a distinctive sound between the ii (which is minor) and the II7 chord which is Major.  There's also a distinctive sound when it resolves to the next chord.You can go further into the secondary dominant chord concept by utilizing the V of the V of the V. In the key of C Major it would be A. The V is G. The V of G is D. The V of D is A. Adding the flatted 7th to A would transform it into A7. And as you know, A is still the 6th tone.