Secondary Dominants & Borrowed Chords
A key has one dominant — unless you rent more. Point a dominant seventh at any chord and it briefly becomes a tonic; borrow a chord from the parallel minor and the sun goes behind a cloud.
Builds on: 4.3 Progressions & Cadences5.1 Seventh Chords & Inversions
Every chord can have its own dominant
In C major the dominant is G7, and its job is to pull to C. But the pull of a dominant seventh is a relationship, not a fact about G: any dominant seventh pulls to the chord a fifth below it. So if a song in C wants to make the arrival on G feel like an event, it can precede G with G’s own dominant — D7. D7 is not in C major (it has an F♯), but for one bar it treats G as a temporary tonic. That chord is a secondary dominant, written V/V (“five of five”), and the brief pretence is called tonicisation.
Spelling one is mechanical: take the target chord’s root, go up a perfect fifth (seven semitones), build a dominant seventh there. The target must be a major or minor chord — you cannot tonicise the diminished vii°, because a diminished chord cannot be a tonic.
What they sound like
A secondary dominant introduces one note from outside the key — the new leading tone (F♯ in D7, G♯ in E7, C♯ in A7). That foreign note is a small spark of colour and expectation. V/V is everywhere: the turnaround of countless folk songs, the “II7” chord in ragtime, the D7 that sets up the final G–C in hymns. V/vi (E7 in C) is the most emotional of them, because it leads to the relative minor: the E7–Am move in Creep, in All of Me, in half the songbook. V/IV is the tonic with a flattened seventh (C7 → F), the sound of the first change in a blues.
The other way to colour a key: borrowing
Secondary dominants add tension. Borrowed chords add shade. C major and C minor share a root, and a piece in C major can reach into C minor for a chord or two — that is modal mixture. The favourites, with their names in C:
- iv (Fm): the subdominant turned minor. IV–iv–I is the classic “bittersweet ending” — Creep again, In My Life.
- ♭VI (A♭): a bright, surprising major chord where vi (Am) was expected. The epic lift in film scores.
- ♭VII (B♭): the rock chord — ♭VII–IV–I is Sweet Child o’ Mine, Hey Jude’s coda, a thousand others.
- ♭III (E♭): rarer, a sudden step into a distant major key.
The trick works in reverse too. A piece in C minor that ends on a C major chord borrows I from the parallel major — the Picardy third, the sunrise at the end of a sad Bach chorale.
A secondary dominant that does not go to its target sounds like a wrong note; the whole point is the resolution. V/V wants V next, V/vi wants vi. (Chains are fine: V/V/V → V/V → V → I walks round the circle of fifths.) Borrowed chords are freer — iv usually goes to I, ♭VII to I or IV — but they still like to come home.
The lab is a progression editor: eight bars, any key, every diatonic chord plus five secondary dominants and four borrowed chords. Green arrows tell you when a secondary dominant lands on its target; red ones when it does not.
🎼 Lab — Secondary Dominant Player
Eight bars in any major key. Click a bar, pick a chord — diatonic in white, secondary dominants in violet, borrowed chords in blue — and press play to hear the loop.
- Load the I–V/V–V–I preset and then swap the V/V back to ii: same bass note, far less pull.
- Put V/vi before vi and listen for the G♯ (in C) — then put it before IV and hear why the arrow turns red.
- Try IV–iv–I in bars 6–8: the borrowed iv is the sound of a bittersweet ending.