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What Is a Secondary Dominant?

What Is a Secondary Dominant?

A secondary dominant is a dominant seventh chord that pulls toward a chord other than the tonic. The V7 → I move is harmony's strongest pull; a secondary dominant borrows that pull and aims it anywhere. In C major, D7 isn't diatonic — but put it before G and it works, because D7 is the dominant of G. Analysts write it V7/V: "five of five."

How to spot (and build) one

Take any chord you're heading to. Its dominant lives a perfect fifth above. Make that a 7 chord and you've built the secondary dominant:

  • Heading to Am (the vi in C)? E7 → Am. That's V7/vi — the sound of "Crazy Little Thing Called Love" and a thousand doo-wop turnarounds.
  • Heading to Dm (the ii)? A7 → Dm (V7/ii).
  • Heading to F (the IV)? C7 → F (V7/IV) — the move 12-bar blues makes in bar 4.

The tell in any song: a chord that "should" be minor showing up major or as a 7 — a D7 or E7 in the key of C is almost always a secondary dominant. More out-of-key machinery in Chords Outside the Key.

Why songwriters love them

A diatonic progression is a calm walk; dropping in a secondary dominant makes the next chord feel inevitable — the progression leans forward. It's one chord of controlled tension with a built-in landing.

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