Secondary dominants: what they are and how they sound

In C major, G7 is V7 and pulls to C. Now take the chord a fifth above D minor: that is A, and as a dominant seventh, A7. Played before Dm it pulls there the same way G7 pulls to C. A7 is V7/ii, the dominant of ii.

A secondary dominant brings in a note the key does not have. A7 in C major brings C♯, a semitone under D, the root it resolves to. That out-of-key note is almost always the new leading tone, and it is what your ear hears as pull.

So on paper, look for a dominant seventh whose root sits a fifth above a chord of the key. By ear, listen for a single bright note that is not in the key, followed by the chord a semitone above it.

C major has five: A7 to Dm, B7 to Em, C7 to F, D7 to G and E7 to Am. The seventh degree is left out, because its triad is diminished and a diminished chord is not somewhere a dominant resolves to.

Each major key has its own page with every secondary dominant spelled correctly, its out-of-key note, its leading tone and the resolution to play.