Secondary dominants in jazz

The definition, and why it is simple

A key contains exactly one dominant seventh chord: the one on the fifth degree. So any other dominant seventh in the progression is, by definition, not from the key. A secondary dominant is such a chord used deliberately: it is the V7 of some degree other than the tonic, and it resolves there.

In C major, A7 is the dominant of D minor. D minor is the ii chord of C. So A7 is written V7/ii, read "five of two", and what it does is borrow the entire pull of a V-I cadence and aim it at a chord that is not the tonic. For two chords the harmony behaves as if D minor were the key. Then it hands the phrase back.

The five you will meet

• V7/ii — A7 in C. The most common of all, because ii is where progressions restart.
• V7/iii — B7 in C. Rarer, and it usually leads to a chain rather than standing alone.
• V7/IV — C7 in C. The tonic itself turned into a dominant. This is why the blues tonic has a flat seventh.
• V7/V — D7 in C. Approaches the dominant from above; often the first chromatic chord a student meets.
• V7/vi — E7 in C. Sets up the relative minor, and often opens a long descending chain.

There is no V7/vii, because the seventh degree carries a diminished chord and nobody resolves to one.

How to find them yourself

1. Find every dominant seventh in the progression.
2. Cross out the one on the fifth degree of the key. That one is just V.
3. For each of the rest, ask what chord sits a fourth above its root. That is its target.
4. If the target is a degree of the key, you have a secondary dominant: name it V7 of that degree.
5. If the chord actually goes there, the reading is confident. If it does not, it may be a tritone substitute, a deceptive move, or the start of a modulation — and it deserves to be labelled as uncertain.

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