What is a secondary dominant?
A secondary dominant is a dominant seventh chord that resolves not to the tonic but to another degree. For example, in C major, D7 is V/V (dominant of the dominant) because D7 resolves to G.
How to build a secondary dominant?
Take the target degree and build a dominant seventh chord (major triad + minor seventh) from the note a fifth above. V/ii = A7, V/iii = B7, V/IV = C7, V/V = D7, V/vi = E7.
- V/ii = A7 → Dm
- V/IV = C7 → F
- V/V = D7 → G
- V/vi = E7 → Am
Usage
Secondary dominants are often used to strengthen cadences, create modulations and add chromatic movement. V/V (D7→G in C major) is the most common case.