>Step by step
Expanding a chord progression by inserting secondary dominant chords
1 Here’s the C major scale – seven notes, from C to B, played on the white notes of the piano keyboard. For the purposes of this tutorial, we’re going to use the Roman numeral system to number the degrees, or notes of the scale, from I to VII. If I is the tonic, or root note of the scale, the fifth degree, labelled V here, is known as the dominant – G in this case.
2 To harmonise a major scale, we need to stack up alternate notes on each degree of the scale to form three-note chords, or triads. Here’s the harmonised C major scale, producing the chords C, Dm, Em, F, G, Am and Bdim. This provides seven chords to choose from when writing, so using secondary dominants is a good way that we can expand on this.
3 In any given key, the I chord is called the tonic and the V chord is called the dominant. In C major, the dominant chord is G major. Moving from G major back to C major makes for a very satisfying move from the tension of the G to release of the C. This move is also known as a perfect cadence. 4 To work out the secondary dominant, we need to imagine for a second that the dominant is actually the tonic. In other words, we need to tonicise the G major chord. Picture the harmonised G major scale, with G major as the tonic chord. The dominant of this scale would again be the chord built on its fifth degree, which would be D major.
5 To see what a secondary dominant does to a progression, here’s a simple diatonic sequence, C- F - G - C (I - IV - V - I). Let’s tonicise the G chord by inserting a secondary dominant (D major) just before it, giving us C- F - D - G - C. Because D major isn’t in the key of C major, the ear is tricked into thinking that the G is the tonic, until the G itself resolves back to the C.
6 We now have C- F - D - G - C. Since the Roman numeral for the dominant degree of a scale is V, the symbol for this secondary dominant in Roman numeral notation is V/V - the dominant of the dominant. So now, in Roman numeral format, our progression reads I - IV - V/V - V - I. But that’s not all – let’s take the idea a little further…
7 There are actually five available secondary dominants in each key - each degree of the major scale has one, aside from the seventh degree. The diatonic triad built on this degree is a diminished chord, which will never sound like a tonic chord, and thus is not eligible for a secondary dominant.
8 Here are the secondary dominants for the degrees of C major other than the tonic and the leading tone, plus the chords that they resolve to. We’ve got V/ii (A major) resolving to Dm, V/iii (B major) resolving to Em, V/IV (C major) resolving to F major, V/V (D major) resolving to G major and V/vi (E major) resolving to Am. Notice a pattern emerging?
9 Though they can be triads, secondary dominants are often written as dominant 7th chords. A dominant 7th consists of a major triad with an added flattened 7th degree of the scale, so in G7’s case, that makes G-B-D-F. Adding this flattened 7th degree increases the feeling of tension we get before the resolution to the tonicised chord.
10 Here’s one last example, in which we’ve got a synth part alternating between C major and E minor. We’re going to tonicise the Em chord, temporarily making it sound like the tonic of the piece by inserting a secondary dominant before it. But how do we work out what the required secondary dominant chord should be?
11 To figure it out, we just need to find the dominant of the Em chord. The fifth degree of the E major scale is B, so our secondary dominant needs to be a B chord. We’ll increase the tension by making it a dominant 7th – B7. In Roman numeral format, this chord would be labelled V7/iii, since the Em chord is the iii chord of C major.
12 With the B7 chord inserted at bars 2 and 6 (having shortened the C major chord by half to make room), the resolution from B7 to Em tricks the ear momentarily into thinking that Em is the new I chord. I’ve gone a step further here and placed a G7 chord before the C major at bar 5 to strengthen the resolution back to the original tonic of C.