Secondary dominants · Music theory
Dominant seventh chords that lead to a degree other than the tonic; how to find them, the related II and chains of dominants.
The dominant of another chord
A secondary dominant is a major chord with a minor 7th that resolves to a scale degree other than the tonic. For a moment, that degree sounds like a provisional tonic. In C major, D7 leads to G: it is the dominant of the dominant, written V⁷/V.
Within the key, only V⁷ has that structure. So whenever a dominant seventh appears on another degree, it also shows through its foreign note: F♯ in D7, the leading tone of G. That note is what gives it strength.