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Dominants of Dominants
Builds on Common Progressions
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Every diatonic chord in a key can temporarily borrow its own 'dominant' — a chord built a 5th above it, made major (or dominant 7th), that pulls toward it exactly the way the real V pulls toward I. That borrowed chord is called a secondary dominant, labeled V/x ('five of x') for whichever chord x it's targeting.
The most common example is V/V (the 'five of five') — in the key of C, that's a D major (or D7) chord, borrowing the note F♯ (not naturally in C major) to strongly pull toward G, the real V chord. This briefly tonicizes G — makes it feel like a momentary 'home' — before the progression continues on its way, usually back to the real tonic.
⚠️ Common mistake
Treating the borrowed accidental in a secondary dominant as evidence of a full key change (modulation). A secondary dominant is a brief, local pull toward one chord — the music usually returns to the original key immediately after, unlike a true modulation which settles into the new key for a while.
📝 Summary
A secondary dominant briefly borrows the 'V of' any diatonic chord, using an out-of-key accidental to pull strongly toward that chord before the progression continues — most commonly V/V, the five-of-five.
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