Secondary Dominants

From lesson one: a dominant seventh chord points at the note a fifth below it. G7 points at C. That pull is so strong that composers long ago noticed you can aim it at any chord you like, not just the tonic.

A secondary dominant is a dominant chord borrowed from another key in order to set up a chord in this one. If the chord you want to arrive at is A minor, you precede it with the dominant of A — an E major or E7 — even though E major is not in the key of C. We write it V/vi and say five of six.

This is the single most common reason a song contains a chord that is not in its key, and it is worth being able to spot.

Here is a real one. This track is in C major, and its chords are C, G, A minor seventh, F, F major seventh — all comfortably in the key — and then an E major.

E major is not in the key of C. The third degree of C major gives E minor, not E major. What the E major is doing is pointing at the A minor: E is a fifth above A, so E major is the dominant of A minor, and it makes the arrival on A minor feel like a destination rather than just the next chord.

E

E major in the key of C. Not a diatonic chord — the G sharp inside it is foreign to the key, and that note is exactly what pulls towards A.

Am7

A minor seventh, the chord it is aiming at.

A rock track in C major. Its chord vocabulary is C, G, A minor seventh, F and F major seventh, plus that one E major — listen for the moment it arrives and where it goes next.

Once you see the mechanism you start noticing chains of it. Because every dominant points a fifth below, you can line several up and let them fall like dominoes.

A blues in G that runs E, then A7, then D7, then G is doing exactly that: E points at A, A7 points at D, D7 points at G, and only the last one is actually the V chord of the key. Every chord in that chain is the dominant of the next, which is why a progression full of foreign chords can still sound completely logical.

Two practical notes for spotting them. A major chord where the key says there should be a minor one is a secondary dominant nine times out of ten — that is the giveaway. And the chord it resolves to is almost always a fifth below, so if you are unsure which chord is being targeted, look at what comes next.