Secondary Dominants

Every major or minor triad in a key can temporarily act as a miniature tonic, and get its own dominant pointed at it — a chord borrowed from outside the current key for one purpose only: resolving strongly into the chord that follows it. This is a secondary dominant, written V/x ("five of x").

The most common by far is V/V — the dominant of the dominant. In C major, V is G major; the dominant of G is D major. So a D major chord (containing F#, a note outside C major) resolving to G major is V/V-V — it makes the arrival on V feel like its own small cadence, adding weight right before the real dominant.

You can point a secondary dominant at almost any diatonic chord: V/ii, V/vi, V/IV are all common, especially in jazz and pop harmony, where they're an easy way to inject one chromatic note (that borrowed leading tone) without leaving the key's overall sound. The target chord doesn't need to be major — you can tonicize a minor chord (like vi) the same way, since the secondary dominant is always major regardless of what it's aimed at.

Secondary dominants are the gateway between diatonic harmony and full modulation: use one and return home immediately, and it's just color. String several together, moving further from the original key, and you've started to modulate — which is exactly the next lesson's topic.